It's a bit of a pity that the SCTA and FIA don't share common engine displacement classifications, but you are right - at 1506 cc in '59, and 1489 in '57, it would be considered an H motor by SCTA today. Looks like the discrepancy is based on rounding off CI and CC to the nearest 1/2 liter.
The story is that Hill actually tested the car in '57 preparing for the celebrity drive by Moss, and that Hill had actually gone faster. Given all the press that MG was counting on to promote the new Twin Cam MGA both in Europe and the United States, it wouldn't do to have some young upstart take the laurels from the venerated Sterling Moss.
Never let the facts screw up a good story . . .
Moss' best speed in '57 was 245.64 in the flying kilo, 245.11 in the mile - Hill in '59, 254.91 in the kilo, 254.53 in the mile.
Two other significant differences - in '57, the car ran a fin and in '59, it was removed. The other difference was that Moss ran in August - Hill in October.
Would have liked to have met Phil Hill.
By the way, Mark - I have an e-mail in to Huffaker with a link to the video. Let's see if they get back to us.
UPDATE -
Just heard from Joe Jr. Sr. is not in the clip.