Samuel Taylor Coleridge's use of ‘must have’

In an earlier post I considered Mervyn Peake's use of the modal must in phase-modified clauses.  The ‘must have V-en’ construction usually encodes an inference as to fact (‘What happened to my agent?  Bastard must have died’), but Peake also uses it where I'd expect the modal would, e.g. ‘had he not made this same journey through the darkness a thousand times he must surely have lost himself in the night.’

Well, I've just stumbled across the same phenomenon in Coleridge's description of his death-defying descent of Broad Stand in 1802:

‘the Ledge at the bottom was [so] exceedingly narrow, that if I dropt down upon it I must of necessity have fallen backwards & of course killed myself.’

A rummage turns up another example from ch. 14 of his Biographia Literaria (1817), discussing the turn-of-the-century Lyrical Ballads:

‘Had Mr Wordsworth's poems been the silly, the childish things, which they were for a long time described as being; had they been really distinguished from the compositions of other poets merely by meanness of language and inanity of thought; had they indeed contained nothing more than what is found in the parodies and pretended imitations of them; they must have sunk at once, a dead weight, into the slough of oblivion, and have dragged the preface along with them.’

And finally, again from the Biographia, in Satyrane's Letters no. 1:

‘And after dinner, when he was again flushed with wine, every quarter of an hour or perhaps oftener he would shout out to the Swede, “Ho!  Nobility, go—do such a thing!  Mr Nobility!—tell the gentlemen such a story,” and so forth, with an insolence which must have excited disgust and detestation, if his vulgar rants on the sacred rights of equality, joined to his wild havoc of general grammar no less than of the English language, had not rendered it so irresistibly laughable.’

Maybe I've just led a sheltered life when it comes to this construction.