We have a group that regularly meets on 146.56 FM simplex.
On occasion we get interference from 146.55 FM which seems to be a popular freq for some reason.
According to my understanding 146.55, although not illegal to transmit on, is definitely not a legitimate freq according to the ARRL band plan.
I've already told this to some hams using that freq and now have a few hams visiting here from Arizona using the annoying 6.55.

With either 15 kc spacing (suggested for the east) or 20 kc spacing (suggested for the west) 146.55 comes up in neither.