I'm not sure what this was used for.
Someone suggested it might be used to sub the 7DP4 in a 621TS but the shape is all wrong.

http://cgi.ebay.com/RCA-7CP4-CRT-used_W0QQitemZ170418255066QQcmdZViewItemQQptZLH_De faultDomain_0?hash=item27adb834da

WWII Military TV, The 7CP1 and 7CP4 were introduced in 1943. Equipment like this:

http://www.qsl.net/w2vtm/mil_television_history.html

I have read that the P-1 green was preferred in this application as it offered better contrast.:scratch2:

jr