The on-sale date is from the publication date recorded in the Catalog of Copyright Entries, Part 2, Periodicals, 1941, New Series, Vol. 36, No. 4.
Carlson can be recognized by his horses with rather long, narrow tapering heads, a bit like sea horses, and especially by his way of drawing the legs of running horses. In panel 7 on page 3, notice the outstretched front legs of Silver, and his long narrow head.