To the best of my knowledge, almost all GE 6072s had black plates. There may be one or two exceptions to this (eg there appear to be some made in the final week or so of 1955) where they used grey plates. I can only speculate that they ran out of the material they normally used for the 5-star tubes and had to go with the standard plate material. All other GE 5-stars, JG/JANs, and 6072s produced by GE and rebranded for other companies had the black long plates. If it was made by GE and had a grey plate, then it was probably a shorter plate and labelled 12AY7. Likewise, if you see a grey plate tube labelled 6072 or 6072A, then it's extremely unlikely that it was manufactured by GE. A lot of the auctions you see out there are actually for GE 12AY7s, but they include 6072 in the title or description because they are assumed to be equivalent.
Some RCA 6072s were actually GEs that were rebranded. Others have shorter grey plates and don't look much different than a 12AY7. Pretty much every newly made "6072" looks like a 12AY7.
As for why they went with the black plates for the 6072s, I can only assume that it contributed to lowering the noise level and/or prolonging the life of the tube. But I'm really not sure, and without having one of those mystery late 1955 tubes one can't really make an accurate comparison since there are other features that would also have an impact on the sound and longevity of the tube. There's the aforementioned length of the plate, the addition of a third mica plate (at least until 1963; the disappearance roughly coincides with the switch to calling them 6072A), and the addition of vertical support rods between the plates from 1959.
