Phil Wood is probably your main option for an upgrade, but at least it's a good one. They have stock options of 135 and 145 but considering the bike, I would just order a custom spindle to match what you have exactly. Pull the cranks and measure; don't assume the 138.5 is actually exactly right in this case.
You could also do measurements and potentially play the odds whether the 135 will work well enough. There are questions here about this. Note that sometimes there's an element of not knowing until you try on 1x systems when you move the front chainline in, because it can be unpredictable when issues with the chain wanting to catch when in the smaller cogs can result. Phil square taper BBs are unique in that they can be tuned for this if needed by offsetting them in the frame, but you ideally don't want to do that since it offsets the Q as well.
I don't know why they use this spindle length. My guess is the rear hub spacing is an extra wide, proprietary size. Depending on what's going on with why it's got a 138.5 square taper with proprietary cranks (or at least looks that way) in the first place, there may be some possibility of upgrading cranks as well if you wanted.