Many thanks for your help with this. I completely missed that back to back R's and was looking at the marking in a completely different way. Based on the fact that the bayo was sold out of service then i'm quessing that this may have been due to irreparable blade damage that possibly resulted in it being cut down / repaired. It's also worth noting that none of the of the bayo's blade is sharp in the slightest (I would even go so far as to say that it is rounded / smooth); possibly suggesting that it has been used for some other purpose (e.g. letter opener??). Or given the age of the bayo possibly as a fighting knife and then later as a letter opener (or something similar).

With regard to the profile of the ricasso I've just checked 'Skennertonicon & Richardson; Britishicon & Commonwealth Bayonets (p.168)' that suggests Enfield were the only manufacturer of the Mk1 1st Type. The way in which central rib meets the ricasso (quite gently for an Enfield and comparatively sharply for a Sanderson) would suggest that the bayo was made by Sanderson (who I believe only received contracts for the 1888 bayo following the revised Mk1 2nd Type design and would not therefore have ever made an 1888 with the three rivet grip)

Again, I may be missing something completely obvious here. But any additional thoughts on who manufactured this bayo would be greatly appreciated.