I was watching the progress of a sales item on ebay this week.  It was a Snap-on hex female open-faced reversible ratchet:  R-720.
Based on other R series rats I see on the web, the 20 indicates 20/32" or 5/8".
I see them in 3/8", 7/16", and 1/2" as well.
I assume they were not designed to be used with drive plugs--although some have been fashioned or purchased and pressed into service.
Can you guys give me any other info on these--or are they simply early ratcheting wrenches, perhaps a bit before their time?
What other sizes were made?
https://www.ebay.com/itm/Vintage-Snap-On-Ratchet-R-720-Hex-drive-5-8-1946-date-code/402665492225?hash=item5dc0bbcf01:g:qQ8AAOSwCpVgEyNX 

In the 1980's I purchased a set of Snap-On sockets that fit into a 5/8" hex wrench, using an o-ring to retain the socket.  The chrome plated set had two main features 1) they were low profile, fitting in tight spaces and 2) the bolt or threaded rod could pass thru the socket, allowing it to be slid down a long bolt thread to the nut.  I used them with a 5/8 - 11/16 ratcheting wrench but maybe there was a special wrench meant for the set?
Maybe this wrench is for this application? 
Al