Phill, The last 6 volt Ford vehicle was the 1955 model year. You probably are correct about the length but I will know more when I return home.
The coil on Ebay that you are referencing is listed as pre-owned and not NOS. It just has the correct box. As far as being of 1963 vintage, that is correct as 73 would definitely be Motorcraft. Ford also physically changed the coil to a black top on replacements some time in the 70's but used the same B6A part number for a while. As far as the numbers and letters between the posts molded into the top, maybe that changed some time in 1963, but I will need to check further on that.
-Fred-