These are magazine blocks for Sig Sauer P320/P250 compact and full size magazines. They are useful for limiting the number of rounds that can go into a standard capacity magazine.There are 2 versions here and each version comes in 6 different heights. Both versions replace the magazine insert that is attached to the bottom of the magazine spring.
One version has a peg that locks into the magazine base plate just like the standard factory insert does. The other one has a hole in it that is sized to take a 5/32" rivet with a 3/16 - 1/4 inch grip range available at any hardware store.
Sizes used to convert to 10 rounds:
9mm Compact 15 Round: 30mm to 34mm
9mm Full 17 Round: 40mm
.40 Full 14 Round: 32mm
It is up to you to know your local laws, and to know if this is legal where you live.