I agree its the Umschalter. Tim I spotted your safety lever is stamped with the rare 5A code.
I luckily have 3 early ones bsw 37 and 38 plus a Mauser 39, all have these early features and all are sadly empty.
I believe only the earliest of the production third variants 1936/7/8? left the factory with adjustable rate of fire parts.
But clearly until 1939 may be early 1940? the receivers were capable of housing these parts but weren't actually fitted.