Kurt
I have an Admiral color tv service manual supplement dated April 1957. This for models using the 29Z1 and 29SZ1 chassis. These sets had two large controls (channel selector and on-off volume) on the top right and left above the screen and look completely different from yours: ETF - Admiral Ambassador
http://www.earlytelevision.org/admiral_ambassador.html. It shows 2 5U4 rectifiers in the tube chart. However the chassis shown is horizontal and yours looks to be vertical. The set up controls are in front under the screen and appear to be similar to yours. There was an Admiral prototype chassis # 28Y1 produced in mid 1956 prior to the regular production 1957 models. Some of these prototypes were sold to the public. Yours may be one of these.
-Steve D.