Peter, it is indeed the same as an L1A1 or a commercial Sterling magazine, except for the markings.
There is no date, no Patent information, and it is not marked Sterling.
The only marking in addition to MAGAZINE PATCHETT 9MM 34 RDS is a solitary letter M stamped on the front face of the magazine.
I assume it is an early magazine from when the gun was still called a Patchett but after the helical spring was introduced, so maybe 1952-1953.