I recently sold my Glock 21 Gen4 and I have FIVE magazines left over to sale.
FOUR of them are 13rds and ONE of the is a 10rd magazine I had purchased just for range use, but frankly only used it once or twice. These will work in any generation Glock 21, doesn't have to be Gen4.
All are in like new condition.
Asking $85 shipped to you.