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.
Depending on magazine variances a 9mm compact 15 round magazine will need one of the the 30-34mm blocks to limit the magazine to 10 rounds. A 17 round 9mm full size magazine seems seems to be limited to 10 rounds with the 40mm riser. Your mileage may vary.
Be careful to not break off the little tabs when connecting it to the magazine spring, although it isn't a big deal if you do.
It is up to you to know your local laws, and to know if this is legal where you live.