This bit is based upon both fact and recollection and therefore is subject to updating.
Sylvania supplied a pin-for-pin equivalent of the RCA 15GP22 to Philco. The 15GP22’s were Sylvania branded and came from the large Sylvania tube manufacturing plant in Seneca Falls, New York, which Phillips closed around 1985 after buying the GTE/Sylvania assets earlier in the decade.
Perhaps because Philco was really pissed at RCA for taking the lion’s share of credit for the 1953 NTSC color system in the eye of the public, Philco seemed to avoid RCA 15GP22’s. Period documentation strongly suggests that Philco contributed the quadrature method of combining the I and Q components of the color broadcast signal. Around 1965 I spoke with an engineer who had worked for Philco. The impression I took away from that discussion supports the notion of bitterness by Philco toward RCA over the public’s perceived monotheistic development of color television by RCA.
One of the CT-100’s shown operating on my site has a NOS Sylvania 15GP22 that had been originally sold/shipped to Philco. Here’s a link to an operating Sylvania 15GP22 in a CT-100:
http://home.att.net/~pldexnis/HoF/AndyLee.html
More in keeping with the original question though, according to Chuck Azzalina, the 1953 Philco 15-inch color prototype he restored, and developed a schematic for, was NOT based upon the RCA developmental Model 5, the forerunner of the CT-100.
Quote from Don: "You will note some subtle circuit similarities between the RCA and the original 1956 Philco TV123."
My current restoration project is one of the four known 21AXP22-based 1956 Philco 22D5102 color sets. Much to my surprise after digesting the Philco prototype information from Chuck, the TV-123 (chassis designation of the 22D5102) circuitry mimics the CTC2 in the area of chroma processing, particularly the control-circuit for the chroma reference oscillator. The vertical centering circuit in the Philco is also similar to that in the CTC2 through CTC4. That didn’t change until the RCA CTC5.