Daryl,
* Please read Silvereagle29 (Don)'s post in the following URL. Should answer many Luger magazine questions including yours.
http://luger.gunboards.com/topic.asp?TOPIC_ID=2857
* OBTW: Just as Mauser changed their Oberndorf plant code from "42" to "byf" in 1941 (per Army "request), so too did C.G Haenel change their plant code designation from "122" to "fxo". The "sE/37" is the Army Inspector's acceptance stamp(s).
* Hope this helps.
Respectfully,
Bob