So we have two competing hypotheses: one, that these words were pronounced differently, and two, that basic rhyming rules did not apply to these words, and to them specifically.
Nope, you missed one: that the poet intentionally misrhymed it to be jarring for effect, consistent with the fact that the tiger's fearful symmetry cannot be framed and the alterations in the final stanza.
It's suddenly a lot less obvious which one is more likely, isn't it?
You missed the word 'systematic' in my post. It's the third one from the beginning. We would have to suppose that multiple poets, all independently of one another, decided to misrhyme those words and those words specifically. We have Putnam's example, that shows that Blake was not the only one who did it with those words, and I am sure others can be found.
And in any case, I was referring more to methodology in general than this concrete example. A systematic deviation from rhyme
is conclusive proof that something was pronounced differently as far as any proof in humanities is conclusive.
Edit: this does not necessarily mean that this was so during Blake's time specifically - he might have been emulating earlier examples, like Shakespeare, who also did it. But that it was so some time in the past is very likely.ъ
Edit Edit: May have come of as assholish. Sorry. Your statement that Blake might have done it for effect is not mutually exclusive with my statement that 'symmetry' was pronounced differently in the past. In fact, it might have been a sort of double brainy git move on his part, where he got his jarring effect, but also masked it with a kind of tongue-in-cheek historical reference. That sounds like something he would do, anyway.