When the Conshelf was first introduced it was advertised and it capitalized on using the exact same well proven first stage as the Royal Aqua Master. I believe that the very first Conshelf did not have a back-up ring either (I am not positive about this), but it was added and retrofitted into all of them.
You are correct about this. I got a confirmed original conshelf VI that was never rebuilt and it only had the -006 o-ring in the spring block, with no backup ring. The spring block is also different from the later models in that I had trouble actually trying to fit a backup ring in it at all. My conshelf XI and XII have the later style spring block that the more modern conshelf regulators have. I believe that they feature some sort of design difference (height perhaps) to facilitate the backup ring addition.