Welsh,
I would be willing to bet that all of the Nettletons are built on the larger frames. The early ones certainly were because they would have been made prior to USFA's introducing the "Old Armory Original" size frame. USFA web site lists the price of the Nettleton at $1695. List price for the "original size" frame, with walnut grips (like the Nettleton), is $2425. If the Nettleton was made on the smaller frame it would probably be priced closer to the $2425 figure.
I think their use of the phrase, "Old Armory Original," most likely refers to their more expensive "armory blue" finish as opposed to their standard "dome blue" finish. Makes it a bit confusing!
CJF