That's rad! I'd love to build and ride that bike! But I just don't ride 20's anymore. I've got my 24" Widow and I'm building a 26" Bassett right now and that's it. If I ever go back to 20" I'll be looking for a Mack SL. :D
Until we get a serial number I'm not sure if this is a S&M 24"(pre-Widow) or a non gusset Widowmaker. It appears the rear wheel is mounted at the very end of the dropout, which could mean that this was from the batch that had rear end clearance issues.