This is a very confusing area to me.
According to a couple of French suppliers those AM9s are still out there and available, in 4.10 x 19
See eg
https://www.centralepneus.fr/pneu-moto/avon/universal-rr-am9/4.10-19-61h-1483191?adref=%2Fpneu-moto-4.1---19%2F%253Fbrands%25255B0%25255D%253D56Others here in France also
say they have them.
I got a couple of them last year from Vintage Tyres, but they then disappeared from the web site and haven't come back. They don't have any AM26 Road Riders either it seems from looking today, which although a bit of a modern profile for our sorts of machines, are I think pretty darn good and last well too.
It's getting difficult. Vintage Tyres have said 'good news' and that the full range will be available, the bike ones coming from Nova and made in the EU after the factory closure in the UK. Maybe that's where the ones referred to above in France come from. Just don't know. I guess Vintage Tyres' Melksham factory stock has now gone, and new supplies are proving less easy than was hoped (
https://www.vintagetyres.com/blog/avon-tyres-good-news.html)
Or maybe not good.
Meantime . . .Dunlop K81 TT100s for similar look and performance on the SS? Musky likes them and he doesn't hang about, nor care about the
short life! At sub-Musky speeds I haven't managed more than about 2500-3000 miles on them on the rears of anything, but they are in keeping. K82s might also be out there, haven't looked, and then there are the Mitas and others mentioned in this and similar threads.
.