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@retrodicorsa
The definitive guide to what indexes what and how is here:
http://www.ctc.org.uk/Default.aspx?TabID=3946
The variables are the shifter (cable travel per click), rear mech (actual mech travel for each click) and the block or cassette (spacing between sprockets).
'Most' 7-speed blocks/cassettes are interchangeable wrt spacing, but 7spd indexing relies on 5mm c2c between sprockets, so from largest to smallest should be 30mm total, measure your Regina and if it is in the ballpark (+/- a gnats) you should be OK to use either Shimano or Campag but the mech should ideally match the levers, though that said I have never tried indexed Campag downtube levers with Shimano mech or vice-versa... the Shimergo article is particular to Ergos/STIs but the theory and maths applies to downtube lever indexing too.