Yes indeed... that is correct. I'm finding that there are many subtle and a few not so subtle changes between the 66' and 67' Belvedere's... apparently, they are both one year cars and sourcing parts for the 66' has so far proven to be challenging. I'm currently searching for a gauge cluster and found one in a 67' and it looked like it was the same... but slightly different and won't fit. Some other differences I've discovered between both years are the Front bumper, Grill, fenders, steering wheel...and as you stated the side mirrors. I'm sure there are plenty more that will crop up by the time I'm through with this endeavor.OK, this is interesting...
My research indicates that ALL 1967 B body wagons got a pass side door mirror.
Looks like that might not be the case for 66.
Gauge cluster should fit.
IIRC the only difference is the Plymouth emblem, or lack thereof.
67 has it ever so slightly better because a few repro parts are made because of the GTX.
1967 changed to collapseable steering column, which is larger diameter than '66, so dash and chrome trim around column hole is different. Also went to standard 4-way flashers. Little known fact is that Coronet and Belvedere wagons shared the shorter Plymouth wheelbase. Might have had something to do with not having to make longer roof stamping for typically longer Coronet.