It's a soldered on custom made spacer to take the front base of a post-WW2 Steyr side-swing mount, now missing, on the barrel reinforce instead on the receiver ring. The rear mount base is still there, but don't ask me where to find a proper front base or rings to fit. These mounts are out of print since the 1970s and unoptainable now. My advice: remove the spacer and the rear base and start anew.