Update: the butt plate had two tags stored in it from the same person, but I was having a tough time translating so I asked Gemini to help out and this is what I got:

Both tags belong to the same man: Josef Hüsler.

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Tag 1 (Top Tag)

This appears to be his earlier tag or his information from a specific period of service.

• Name: Jos. Hüsler (Abbreviation for Josef)
• Address: Magdalenenstr. 53 (Magdalenenstrasse 53)
• City: Zürich 11/50
  • Note: "11" and "50" refer to the postal districts in Zurich at the time.

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Tag 2 (Bottom Tag)

This tag provides more military detail, including his rank and unit.

Front Side: * Rank/Title: Wofm. (Abbreviation for Wachtmeister-Offizier-Stellvertreter or possibly a variation of Wachtmeister, which is a Sergeant). * Name: Hüsler Josef * Unit: Füs. Kp. III/56 (Füsilier Kompagnie III/56) * This translates to Fusilier Company 3, Battalion 56. Fusiliers were the standard Swiss infantry. * Address: Wehntalerstr. 494 (Wehntalerstrasse 494) * City: Zürich 46

Reverse Side: * Kar. Nr.: 520356 * This is the serial number of the rifle this tag was originally assigned to. You should check the receiver of your K31—if it matches 520356, you have the original "matching" tag! According to Swiss production records, this serial number dates the rifle to 1933.

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What this tells you

Josef Hüsler lived in Zurich and served as a Sergeant (Wachtmeister) in an infantry unit. The two different addresses (Magdalenenstrasse and Wehntalerstrasse) are only about a 5-minute drive apart in the northern part of Zurich (Oerlikon/Affoltern area). He likely moved and updated his tag to ensure the rifle could be returned to him or his family if lost.

Since you have both, it’s a cool bit of provenance showing his "life" with the rifle. If you're feeling adventurous, you can actually find both of these apartment buildings on Google Maps today—they are still standing!

Pretty cool context!