Y3 is 3mm wider in the I.D. measure (where it counts), details here in the OP: https://budgetlightforum.com/t/-/28129. It ain't much but I'll take it! The MaxToch M12 is little more than the Y3 - similar form factor light, but at twice the price - dunno if it's worth it, but I can guarantee the anodizing finish will be better.
ImA4Wheeler - cant' get to that link - direct post links don't work for me anymore. I do have it posted in google drive somewhere... Should be here: https://drive.google.com/file/d/0B1IxYZuk4DjcbzI2S1UyYVpjbGs/edit?usp=sharing
fellfromtree - absolutely worth the driver upgrade, but not replacing it, piggybacking to it, like my icon 
. You have to retain the stock driver because of the driver mounted switch. I simply strip all components accept for the switch.
Oh bout the SR51, $50 is a great price for that light because everywhere I see it listed, it's more than that. I haven't seen one so can't say too much. In a CPF review, the reviewer seemed to measure lower lumens than stated, but for throw - who knows. Really, really weird it's an OP reflector and XML U2 LED - I've never seen a so-called "thrower" with an OP?? What gives there? My Y3 throw measurements taken from 5 meters vary from ~52 to ~61 kcd, depending on the unit and batteries, etc... Maybe I include slight dabs of TLC (Nyogel, etc.) before I test, but usually my stock #'s are as stock as possible. Some say I'm high all the time - maybe? I wish...