Thats a west coast B model. Something we rarely see on this side of the country.
What is the difference?
Western trucks often had larger motors which needed larger radiators. The width of the grill/radiator and the exposed radiator tank is the big give away.
When the R models came out there was several differences, mainly grill size and hood length. Many of the west coasters where long hoods with long frames, frames in the 200+ inches to make bridge laws. Some of the fancy west coast R models even had a bulldog wearing a cowboy hat stitched into the door.
The last Mack our family owned was a 77 west coast RL model pure bread Mack. It had the Gold dog on the hood and the dog wearing the cowboy hat on the inside of the doors. I barely remember anything about that truck, just those couple details and it rode like hell............absolutely the roughest riding truck I have ever ridden in. I remember bouncing out of the seat and hitting my head on the ceiling or my face off the dash several times. I had a chance to buy it at an auction a few years ago but it just went to high.
https://www.bigiron.com/Lots/1977MackRL700LSTTATruckTractor