Byte 6 is now converted from 0x53 to 0x7f when searching groups
register.

0x7f is the abstract value for byte six of the sets & packs register. An
actual ul in an actual mxf will never use 0x7f. Everything I've seen has
used 0x53, which indicates a local set with 2-byte tags.

Consider AudioChannelLabelSubDescriptor. This is the ul defined in SMPTE
ST 377-4:

060e2b34.027f0101.0d010101.01016b00

But in an actual mxf, you'd see:

060e2b34.02530101.0d010101.01016b00

And if you copy from an actual mxf, say, after using Cinecert's klvwalk,
you'll see no results. Because the abstract form is in the regxml, while
a concrete form is found in mxf headers.

There are other values that might be worth transforming, listed in ST
336:2017 § 7.4 Local Sets, Table 5. Will leave that for the future,
since (a) it requires a much larger change (b) I haven't seen any yet
besides 0x53.