1st pass thru the Steyr proof facility, being a satellite Vienna stamping station, in 1927 w/ the scattergun having a diameter of 15,5 mm and the solid projectile tube @ 7,3mm. Final pass in 1927 was 16,2? and 8,0mm. I've seen that open ended infinity symbol as well as the script M many times.

Cheers,

Raimey
rse