Hi there buddy
Hate to throw a spanner in the works but I recognise the cable guides from my "3 Raleighs". Not saying this is a Raleigh but most likely English built / modified.
My 5 Columbus bikes have a different style cable guide, all the same on all 5 bikes. They are simple like a single nipple with a groove in the centre, not split like Reynolds ones. Perhaps the clues lie there ? It's also very heavily pitted. Columbus tubes especially SL SLX which are 80's tubing don't really suffer that sort of heavy rot, even when they have been left out for donkeys years.
Hope some of this helps. If its an Italian BB shell, then perhaps it IS an early 70's Columbus with modified cable guides. Got a bike here exactly just that, a 70's 531 frame with typical 80's cable guides, added at a later date (only know this because when I stripped the frame back to bare metal it was noticable).
1 last tip : tubes are often individually marked with there correct status ie: 531/531c/Columbus. Try looking near the lugs on the tubes themselves. I ID'd 1 of my own by shear luck this way. Found the 531 stamp on the actual tube. After close inspection the other tubes had them too, faint but there, right near the lugs.
Good luck, yours Laz.