Thanks for the compliment. There's a lot to like about this TV.
I corresponded with Tom Albrecht about the CRT type, and his conclusion is that DuMont began with the 17AP4 but soon switched to 17BP4. On my set, there's a rather crude hole in the back cover, to make room for the slightly longer 17BP4 CRT. No dome over the hole, however.
On his set, there is a shallow dome covering that hole, and the tube label inside the cabinet indicates 17BP4.
I'd like to get a strong 17BP4A some day. Mine's watchable, but weak.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html