Hi Hamish. The bike came with an M33 tank when I bought it. I did run the bike with a chromed B31 tank at one time. This you had to alter. I did that by welding new front mountings onto the old ones, otherwise the tank sits to high and fouls the handlebars. It sprung a leak so I reverted back to the one that came with the bike. Ian

You can see that the eBay bike also has a modified B31 tank but still a very nice looking machine.
Richard.

Very nice all these OHV's. Before the war they even had twin port heads, if I'm not mistaking.
Maybe even with upswept pipes (not sure..)