My '71 K20 burb has a rear heater under the 2nd seat. From what I've seen, all burbs have the two holes in the floor for the rear heater hoses. My '71 K10 burb has the holes but no heater. Here in Montana, that rear heater is
really appreciated.

Comes with a cylindrical pull switch on the dash at the left lower corner thru the metal.
The interiors came in different trim. A well optioned burb should have side panels all the way back save over the spare tire cavity. Any carpet can cover the entire burb to include a separate piece over the wood floor. Jack & handle should be in the spare tire cavity. Tinted windows was another option.
Don't forget seat belts for all positions. Might not have been mandatory, but they could have them all the way back as do my C10 and K20 burbs.