The first number is the size of the bore and the second the amount of constriciton. In a 12 gauge the nomingal bore is 729 but a given gun might be somehat more or somewhat less. In a 12 gauge a constiction of 10 would be IC and 20 modified as an example. A given gun might be 730 as opposed to the theoretical 729 and a constriction of 18 would mean the acrual choke was 712 with a constriction of 18 in the 730 gun whereas a 729 gun with a constrictuion of 18 would be 711.Your numbers appear likely to have come from a 20 gauge.
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