The "5" prefix preceded the "S" series of model numbers. It is believed that the "5" indicated the five-tube design.

5-T
5-T "Sky Buddy", 5 tube 3 band single conversion receiver covering .54-18MHz, IF 465kHz. BFO. Production year 1935-36. Supplementary photos qa

5-T with tuning indicator
5-T "Sky Buddy", 5 tube 3 band single conversion receiver covering .54-18MHz, IF 465kHz. BFO. This radio has tuning indicator, not described in Chuck Dachis book. Production year 1935-36. qa

51C2A
51C2A Clock radio, 5 tube AM band, IF 455kHz. Production year 1954.
Supplementary photos qa

51C2B
51C2B Clock radio, 5 tube AM band, IF 455kHz. Production year 1954. qa