I was going to recommend you take a look at Doug id's rebuild thread, but he's saved me the bother
They're a nice looking tyre, and are probably the ones my 550 will get to wear after the rebuild
My 550 is currently wearing Metzlers, and they have a good 'period' looking tread too, they just aren't to clever in the sticking to the road department
I agree with 736cc, The rear on that triumph looks just like the tyres I had on the front of my 'cal-look' beetle, Firestones if I recall correctly.
Starrion, getting a 15" rim is possible if you do a bit of lateral thinking, i.e: don't use a bike rim, many older British cars, Jags, MGs, etc, ran with 14 or 15" spoked wheels. If you can get an unpunched rim from one of the many companies that remake them, you could quite easily get it drilled to suit a bike's spokes and hub. I'm not saying that's the way he's done it, or even that it is a 15" rim, just that it could be done