That is a 16gp4 in that one almost certain of it. It's too large to be a 12, and the only round 16" RCA sets that I have seen are 16GP4 tubes.
here is a link to my TC166 with the 16GP4
http://antiquetvguy.com/Web%20Pages/...PhotoPage.html
The RCA 8TC721 uses a 16AP4 but I have never seen a 16AP4 in a console like this.