They were published and sold by
Western Stamp Collector in 1938/1939. They started with a series of 20 which was continued through number 40.
John Coulthard, philatelic artist/cachet maker did the drawings
It is difficult to put together a complete set, although not expensive, a complete set of 40 recently sold on
ebay for $39.95.
There appears to be a single addition to the set of 40, a label numbered (00), but I don't know how it fits into the original series.
I also collect these labels on cover postmarked from the towns they represent. Also a challenge to complete.
DonSellos