Yes, the shaving might identify a horse as "post", but might not be visable at a great distance.

My first thought about harness and tack was that I couldn't see a rationale for having a special style of construction. However, the leatherwork could be stamped, embossed, or coloured in a certain way. Particular ornamentation could also be on, or suspended from the leather.

That got me thinking. If the horses and their gear were going though a lot of different handlers, they might want a way of matching up the harness to the appropriate animal. If the horse was marked by letters (for the company) and a number, the leather could bear the matching number (stamped, painted, or in brass). The figures wouldn't need to be big, but they might be large enough to be made out.

Perhaps, for further identification each horse might carry the equivalent of a dog-tag as part of their tack. I'm thinking a small brass plate with the owner's name and address.

As mentioned, just a guess, and something else to obsess about. ;-)

"Mommy, that strange man is mumbling about post horses - what's a post horse?"