Hi David
I tried to post a reply last night but it vanished into the ether. I wondered if perhaps as a newby my posts are moderated and that takes time? Anyway, I'll try again...
The suggestion of a particular regiment based on the French horn is a great idea (I'd forgotten that detail) but I'm not convinced by the KOYLI attribution. Every version of that I've seen or could find on the web last night has a White Rose in the centre. Perhaps the 105 is more plausible but why not 105 as per the cap badge? The Highland Light Infantry also used a French horn, albeit on the usual Scottish background and with HLI in the centre.
But, browsing around, I did find one regiment with an 'empty' horn - the Royal Westmoreland Light Infantry Militia. (But the wrong way round?) Have a look here:
and scroll to the bottom
The top level of the site has a discussion forum which might be worth asking for help on?
Re the shortening of the blade - my guess would be that it got broken and repointed.
Is there really no trace of etching on the blade that might help?
Regards
Paul