No this is the whole point!:roll:
The freehub body isn't longer.
Because 6 speed has the cogs further apart, a 6s cassette fits in exactly the same space as a seven. There has never been a different freehub body length for 6s or 7s. The only difference is that uniglisde ones fitted the cassette with a screw-on smallest cog, whereas the later hyperglide (7s and up) ones had a lockring. By the time HG came out, UG was obsolete so there were no HG 6s cassettes made. So you have a 7s freehub with a custom made 6s hyperglide cassette.
Which would be convenient were it true. From your measurements a seven speed cassette will measure up at 31.85mm outside edge to inside edge, a six will measure 28.35mm, assuming a 1.85mm sprocket thickness. Lets call it a 3mm difference, which is curiously enough the same as the difference between a 7 and an 8 IIRC. To fit a 7 speed to an 8 speed body you need a 3mm spacer. Shimano only got wise to keeping the casette width the same when they moved from eight to nine speed.
There is no additional spacer on my setup.
Furthermore sprockets 5 & 6 have built in spacers so if they came from a seven speed unit they would need additional spacers to index properly, they do not have additional spacers and they do index correctly. Obviously number six can't have come from a UG set because it's not threaded, it's splined. Converting from splined to threaded wouldn't be too hard, the other way wouldn't be so easy.
Shimano did make quite a few parts that were only ever available to OE manufacturers, not retail. Among them were cassettes with unusual combinations of sprockets. Maybe this is another of those strange items available only to OE? Maybe it made sense to some manufacturers.
I don't know what bike this was originally intended for as it's not the original wheel.