Visually, Rockauto has something the shape I'm used to seeing (with the longer ear) made by Standard Motor Products bearing the SMP part number DS40 (listed by RockAuto as replacing Chev/GM part number 1997009 (for cab 13) and 1997002 (for cab 11/12) see parts group 2.459. Rockauto sells them for $13.54 each.
https://www.rockauto.com/dbphp/x,cat...UCTS_DS40.html
A quick look at ebay shows pricing as low as $15.94 including shipping to over twice that for SMP DS40.
Looking in the C15A-04 parts book, it also refers to "after jobs - toggle type mounted on tail & stop lamp bracket". I don't recall seeing this setup in life but think I've seen photos. Also, Phil Waterman posted some drawings of tail light brackets that included a 1/2" OD hole between the light mount holes that would fit a toggle switch for selection between tail and axle lamps.