I recently saw a Myerchin. It must have been the larger model as it was substantial in size and weight. I understand this is the only knife where the blade & fid both lock. It did have some rust, but looked OK. It was being carried by a Salvor.
I understand that some knives have the blade lock and some have the marlinspike lock.
I have always wanted a fid, but never got one. I have a stainless steel ice pick that is relatively thick at the tip, that I use as a substitute. Other possible substitutions, if used with care, might be a philips headed screwdriver, or the awl on a SAK.
One of my friends is a fan of inexpensive ones, as they always seem to end up in the drink.
Boye makes a Cobalt serrated sheeps footed folder that won't rust, but alas no fid. The French steel X15TN would probably be ideal if using a steel, as from everything I have read, it is very rust resistant. Boker uses this X15TN in a couple of otherwise indifferent knives--at least I have not seen any BF members lauding the fit & finish....
I would like to try on in X15TN, especially if the knife has a fid, but have not. It apparently is at least in the good category amoung knife steels, but I have not seen much user info. Apparently it is used by some high end surgical tool makers.