Rarity. No really, that's it... Okay, so technically the baseplate is slightly different being in the style of the older RFB but they shave exactly the same so its nothing to lose sleep over, I just think they look nicer
The slight difference in the weight of the head is made up for by the very small differences in the handle (which is very slightly longer than a normal Fat Handled Tech, hence the balance is not noticeable.
They're great razors and the shaves between the two are identical.
Gillette Old - Mr Razor has it as the 1921-28 Old Type Set 56g. It's gold plated two-piece, a bit light and short handled but if I could only have one it would satisfy both my performance and aesthetic needs.
If it could be any single razor rather than one I already own then...?