Its generally accepted that serial numbers were not applied to magazine bottoms around the end of the u-block in 1941.
The Haenel manufacturer's code was changed from 122 to fxo beyond the p-block in 1941.
Between these two points fxo magazines had aluminum bottoms which were either serialized or not. Both are found.
The black bottom fxo magazine is seen generally beyond the u-block in 1941 and carries forward into 1942. These were typically not serialized.
There are a few examples of serialized black bottom fxo magazines but these are the exception rather than the rule.
All of the above are norms because gun manufacturers are notorious for not wasting anything so if they found some older issue magazines they fed them into production. So exceptions to the above rules are to be expected. For example in 1942, one occasionally sees a type 5, blued bodied aluminum body, un-serialized magazine with a 42 byf.
Hope this helps.
John
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