The Guernsey catalog has a brief summary of the artwork and indicates it was all original illustration. It's all uniformly outstanding:
Like I said, there are a lot of antique autos, one of which (fourth down on left) looks like the background color block has been inserted. It hasn't though, as the actual card shows:
I like the lonely little VW Bug below:
None of this artwork solves the mystery of the "high high numbers" in the set, running from nos. 171-180 and which are of a slightly different composition than the previously 170 cards. The distribution method of these last 10 cards is also a bit of a mystery, as is the fact they can be found with red or blue backs, when all 170 that came before were red.