The act- and rule-utiitarian distinction
It does not follow, because Richard Brandt first formulated the distinction, that therefore utilitarian philosophers had not recognised a distinction long before. Their recognition can be shown by their general discussion of ethical matters. Such is precisely the case with John Stuart Mill. He very evidently possessed the concept of a distinction between act- and rule-utilitarianism. Mill, I suggest and argue, regarded act-utilitarianism as providing the theoretical moral standard.
John Stuart Mill ethical theory
Take Mill's criterion of morally right action as given in Utilitarianism (1862), ch. 2. Utilitarianism is introduced as :
The creed which accepts as the foundation of morals, Utility, or the Greatest Happiness Principle...
holds that actions are right in proportion as they tend to promote happiness, wrong as they tend to produce the reverse of happiness. (https://www.utilitarianism.com/mill2.htm.)
Unless one has already decided otherwise, Mill's idea here appears to be that we should evaluate - morally assess - every possible action in terms of its (probable or foreseeable) consequences and in our final deliberation do that action which is most likely to produce the greatest happiness or maximise what he elsewhere in the same chapter terms 'the sum total of happiness'.
Mill's theoretical standard was almost certainly one of act-utilitarianism. Evidence for this is provided his letter to the Cambridge logician, John Venn, in 1872, in which he states:
I agree with you that the right way of testing actions by their consequences, is
to test them by the natural consequences of the particular actions, and not by those which would follow if every one did the same. But, for the most part, considerations of what would happen if every one did the same, is the only means we have of discovering the tendency of the act in the particular case. (Italics added.) (J.S. Mill, Later Letters of John Stuart Mill 1849-1873, ed. F. Mineka & D. Lindley, in J.M. Robson (ed.) Collected Works of John Stuart Mill, Toronot: Toronto University Press: 1881.)
In other words, while the theoretical standard is furnished by act-utilitarianism, we have to rely on rule-utilitarianism for our practical moral criterion. As if the quotation above were not enough to make the point, the utility of following rules, and the practical necessity for doing so, is present by the strongest implication when Mill considers an objection to utilitarian deliberation based on consideration of time:
Again, defenders of utility often find themselves called upon to reply to such objections as this - that there is not time, previous to action, for calculating and weighing the effects of any line of conduct on the general happiness. This is exactly as if any one were to say that it is impossible to guide our conduct by Christianity, because there is not time, on every occasion on which anything has to be done, to read through the Old and New Testaments. The answer to the objection is, that there has been ample time, namely, the whole past duration of the human species. During all that time, mankind have been learning by experience the tendencies of actions; on which experience all the prudence, as well as all the morality of life, are dependent. People talk as if the commencement of this course of experience had hitherto been put off, and as if, at the moment when some man feels tempted to meddle with the property or life of another, he had to begin considering for the first time whether murder and theft are injurious to human happiness. (Italics added.) Utilitarianism, ch 2: https://www.utilitarianism.com/mill2.htm.
This passage is reinforced by another:
In the case of ... things which people forbear to do from moral considerations, though the consequences in the particular case might be beneficial - it would be unworthy of an intelligent agent not to be consciously aware that the action is of a class which, if practised generally, would be generally injurious, and that this is the ground of the obligation to abstain from it. (Italics added.)(https://www.utilitarianism.com/mill2.htm)
Even these practical concessions to rule-utilitarianism, through the references to what we have learnt 'by experience' of 'the tendencies of actions' and 'if practised generally', is qualified by an acknowledgement of the inherent inadequacy of rules to apply without exception:
It is not the fault of any creed, but of the complicated nature of human affairs, that rules of conduct cannot be so framed as to require no exceptions, and that hardly any kind of action can safely be laid down as either always obligatory or always condemnable. There is no ethical creed which does not temper the rigidity of its laws, by giving a certain latitude, under the moral responsibility of the agent, for accommodation to peculiarities of circumstances (Utilitarianism, ch 2: https://www.utilitarianism.com/mill2.htm.)
What must be accommodated but an act-utilitarian decision?
John Stuart Mill: On Liberty
It is plain that the liberty and the harm principles in 'On Liberty' (1859) are rules. Mill defends, for instance, a rule of free expression. He definitely does not want us to examine opinions one by one and decide whether to allow their free expression.
'On Liberty' is, then, heavily weighted towards rule-utilitarianism but even here he acknowledges the above 'peculiarities of circumstances' by virtue of which his rules might cease to apply. The 'bridge' example in ch. 5 ('Applications') is a case in point.
In more recent times, on the evidence of "Extreme and Restricted Utilitarianism", The Philosophical Quarterly, Oct. 1956, pages 344–354, J.J.C. Smart defended specifically act-utilitarianism. See also his essay in Utilitarianism : For and Against (co-authored with Bernard Williams; 1973).
J.S. Mill On Liberty (1859)
J.S. Mill, Utilitarianism (1863)
A.E. Fuchs, 'Mill's Theory of Morally Correct Action, The Blackwell Guide to Mill's Utilitatianism, ed. H.R. West, Oxford: Blackwell, 2006: 139-158.