And why do you need to know that the vehicle is incapable of traveling faster than 25 mph... so you don't drive into it. Period. If driving into slow moving vehicles wasn't a problem, then there would be no need to have the signs. It's called root cause - the sign is not to just say "I'm moving slow", it's to say "I'm moving slow and you need to be aware and avoid hitting me".
The mailbox on the shoulder of the road is incapable of moving faster than 25 mph and should be avoided as well - if you want to split hairs on it. :2cents:

:stirthepot:
As for the stop sign, it's not meant to indicate anything other than stop - which I guess would be OK if I wanted everyone to stop at my mailbox rather than just avoiding hitting it.
I will note, I don't use them myself on anything but my tractor, etc., and have regular round reflectors on either side of the culvert down by the road.