Wow, didn't know that.
But at least the freehub body is the same size, so it's easiest just to run it as 7s and be done with it. I know that there are some weird 6s freehub bodies around too.
Shimano 6spd uniglide have the built in spacer on the lock ring cog at 2.65mm then just add a 1mm spacer to knock it up. I have just managed to get a proper Uniglide cassette again too realise it had 7 speed spacings I could use my original 6 speed spacers but not the most important lock ring!
So yeah i ground it down by .35mm on a surface grinder at work
cost me a grande capuchino though
Its a pity you can't buy 6 spd though there must be a market
I tried using a 7spd cassette on friction but I hated it and much prefer the indexing system.