Just noticing that the OP's frame has an integral derailer hanger, good thing that the shop didn't cut it off.

I wonder if it's a Campy-style dropout or Huret-style?

Also just now occurring to me that this isn't a US-spec bike and so may have been all original?
In which case, the Allvit mounting angle and cage may have been fully correct for the gearing.
Finding a long-cage Allvit with suitable mounting hardware for the particular style of dropout might then be a worthwhile endeavor.
If the shop has weekly trash pickup, the original derailers might still be in their trash bin/dumpster, fwiw.