According to ePer there are two part numbers 77365542 up until Lucas calipers were fitted "with warm stuck sheet metal on friction gasket" (which I assume means the anti-squeal backing material) and 77362087 after that. However ePer also shows the "classic line" pads 71770989 which are the same for all calipers, so I assume this means all pads should fit all calipers.
I've never seen this problem with ill-fitting pads, but have noticed the difference in chamfers between different (aftermarket) suppliers. Did you measure all the pads to see if they were identical, was the problem with the caliper itself?
The difference in slider pins has been noted before, I've seen the grooves and "shelfs" (you mean there's a bit missing a bit like a letter "D"?) I've never tried to deliberately swap them, and perhaps I've been lucky putting them back in the same position, but again eEper gives identical part numbers for both.
Edited by DaveG (11/05/2018 10:10)
Edit Reason: Added classic line part number
2000 Pearl White Plus & 1996 Portofino 20vt
1985 & 2016 2017 Fiat 124 Spider + XF Sportbrake