It is truly a mixmaster.
A grip safety with a lug almost has to be an ex-Navy pistol; commercial grip safety pistols had no lug at all- they were "slick backs". There appears to be evidence of previous safety markings that have been altered, which could indicate a modified first type Navy frame.
The 1915 dated receiver is military and would have been in 9mm when made.
When or who put it together is not likely to every be known with any certainty; I'd discount completely the Swiss story; though anything is possible.
JMHO.