I may have lead people astray.
I decided to drill the rivets out of my BX15 today to clean it out.
Luckily I put on some 4-power glasses to do so and noticed that the rivet head almost looks like a socket-head screw. A comprehensive set of imperial and metric though did not fit, either they are rivets, or the head has been drilled out after installation...but I thought to try a Torx bit as well, and it worked.
They appear to be T10 Torx-head and it opened up very nicely. Mine is prior to them being restricted to 10rds.
If you have a 10rd BX-15 check it as it may be riveted to prevent it being unrestricted.
But if it is not riveted you can open it up and remove the restrictor block - provided it is legal for you to do so. If you own a Ruger Charger I would recommend you do not modify a restricted BX-15 to avoid being in the position of possessing a 15rd magazine for the pistol. Only pistols are restricted to 10rd mags in all states so it is legal to have .22LR 15rd rifle mags.
These days with the "precursor laws", an empty plastic bottle is a precursor for a Glock pistol and an empty aluminium can is a precursor for an AR15.