Hi John,
The Vintage Motor Cycle Club has an enormous library that has all the info you will ever need. The library is open to the general public, so you don't have to be a member to get anything you want from it.
The VMCC also has a website that lists all their library holdings so you don't even have to move from your seat to find out exactly what Excelsior info they have.
Check them out. You will get a pleasant surprise.
You'll see I haven't given you the website name. That's because I can't remember it. You'll have to put some effort into this yourself!!
Cheers, Bob