I have seen 3 types of +/M stampings along the side of M1929 Ordnance lugers (note the commerical ones did not receive an Ordnance inspector stamping.). Both shown in the photos in the posting are of the types I have seen and have on some of my M1929's. I would imagine inspector stamps wore out and had to be replaced and new ones may have been ordered with a different style to reduce boredom of using the "same ole stamp"...
Sometime around # 66xxx, the Muhlemann stamping went away and was replaced with a "W+F" stamping. See the attached photo I took off the Simpson LTD site.
...
My guess is that Muhlemann retired (he started at Bern in 1913 and the M1929 ran to the end of November 1947) and this W+F stamping was then used to the end of the M1929 Ordnance run of lugers. This is just a guess on my part...no research done.
Here is a scan of a Swiss stamping chart author V. Bobba has in his book about Swiss lugers :
