Gonna be hard to say.
The FN Browning Trombone .22 rifle was produced from 1922 till 1974 with some 150,000 total made.
There are no known serial number records for early rifles, because the FN factory in Belgium was first captured by the Germans, then bombed into rubble during WWII and all records were lost.
Fewer then 3,000 were imported into the U.S., and my book doesn't even cover those serial numbers.
rc
