If indeed the 1878s used a larger diameter shank than the m1917 et al, it is possible that most anyone having a orphan rectangular Lanyard Loop laying around, could have one, and not know it.
It would be interesting to find out when Colt decided to go with a smaller shank.
Assuming the measurement we have so far is correct ( for the Lanyard 'Hole' anyway, on an 1878 ), and not a bored-out-larger example which someone improvised something with, along the way.
If you need one also, can you give us a fairly precise measurement of the existing mounting hole on yours?