Interesting - I have several examples of the 68 pattern without the '68' designation but none of them have the 8415 prefix.
What I do have is a couple of examples of 68 pattern with the last 4 digits 6079 (size 1) to 6087 (size 9). One of them, a size 8, has the designation 'G8 (Size 8) for size. I wondered if this was a specific model for Gurkhas.....
Thoughts?