I've never seen that set with that mask from the factory. I've seen enlarged masks a few times, especially with the early 7" sets. Likely this was done by owners when the larger screen sets came out to make their screens appear a little bigger. On the Net I saw the restoration of an RCA 621TS. It had a rounded mask and the owner cleverly filled in the rounded-out area, refinished it, and it made it look like new.
In fact, here is the link:
http://www.earlytelevision.org/holden_621_cabinet.html
Joe