For what it's worth, the Cuetec website has two specific links and part numbers for 3/8x10 shafts:
https://www.cuetec.com/products/cynergy-shafts/13-962-cynergy-15k-shaft-3-8-x-10-pin/
https://www.cuetec.com/products/cynergy-shafts/13-966-cynergy-15k-shaft-3-8-x-10-flat/
Note the second link has "flat" in the URL. The two links don't seem to give any other definitive indication. Using deductive reasoning though, one might assume that the presence of two links with distinct part numbers might suggest that they intend for the classic 3/8x10 folks to use the first part number and the modified folks the second.
So if you can discern the part number (on the box, maybe?), that may be the answer.