That seems like an odd and rather short spoke length. I vaguely recall that my
650B wheels (Pacenti Brevet rims, similar to older rims like Araya, Shimano 600 tri-color hubs) used spokes around 260-ish mm.
Try Universal Cycles. Lots of lengths in DT (I’m a fan of their Competition and Revolution butted versions), Sapim and Wheelsmith spokes:
https://www.universalcycles.com/search.php?q=Spokes