Two comments, based on personal experience.
(No measurements were made, and are probably not even possible, that would correlate data to experience.
1. From experience with the big, early AKG C12 transformers which had 600Ω, 200Ω and 50Ω secondary taps, I found that 600Ω and 200Ω into a >1kΩ load sounded rather similar, with the 600 having maybe a tiny bit fuller bass.
2. The bigger reason for the U47p's attractive sound may be the good match between the timbre of the early upright transformer and the M7. It may have less to do with the acoustic properties of the two different head shapes. These early pre-BV8 transformers had a certain dryness in the bass (not lack of bass) and a reedy, transparent but not thin mid range.
These comments are really putting a magnifying glass on the subject of the U47p. Having listened to probably hundreds of U47, I find they all are still of one family.