Mechanics and Fixing Any Questions Answered

If you believe my "let's just float a ruler somewhere in the area of the rear hub and make an educated guess" measuring, it's 135mm

But if you read the manuals:
"Horizontal slotted with derailleur hanger and Surly “Gnot Boost” 135, 142, and/or 148mm hub spacing compatibility;"

"As for its 12mm x 145mm Gnot Boost spacing, there are inevitable pros and cons to the system. One the one hand, it allows the use of either 12 x 142mm thru axle or 12 x 148mm Boost thru axle hubs. By using Surly 10/12 Adapter Washers, as I did, 10 x 135mm bolt on or 135mm QR hubs are compatible too, reassuring in this ever-shifting world of ‘standards’. The downside? It’s not a captive thru-axle design, so you’ll lose out on the inherent stiffness of the system."

Ok, this is 2017 version, mine is older and doesn't have this insert rear end, it's just QR so I'm sticking with 135mm.