Hello Olaf,
Good to hear from you again. Thank you for these great pictures! Congratulations of finding such an amazing rifle for your collection!
This looks like a trial or developmental rifle that must have been in the Mauser factory collection. I know that a number of these rare rifles have surfaced from French sources amd a well known German collector recently.
I have only seen only one rifle like this, and in picture form only. Do you have a copy of "Mauser-Gewehre und Mauser-Patente"? If so, look at the patent picture of the Serbian 78/80 opposite page 50 and the German Infantry rifle M.71 opposite page 48. I have been studying these pictures and your photos, and your rifle appears to be a combination of the features and configurations of M.71 and the Serbian 78/89 patent pictures. In addition to the features you mentioned, above, the fore stock, rear sights, and muzzle seem to be still in the M.71 configuration, but the caliber (10.15), receiver tang, cocking piece and triggerguard, and both swivels appear to be the Serbian 78/80 configuration except for the shape of the stock wrist and the position of the upper sling swivel.
Have you disassembled it yet? Is it dated anywhere below the stockline?
Best Regards,
John