Daryl,

Thank you!
After reading your reply I did a bit of Googling for dimensions.
I believe you have nailed it with the .375 Taylor (and its many other names)

The only one of the parent cases I have on hand are 7mmRM.

After removing the cocking piece and firing pin from the bolt I loaded a round in the magazine.
Fed nicely up under the extractor and into the chamber. Bolt closed properly. (BSA U9 w/ Mauser style claw extractor.)

Then fed two and then three rounds through the rifle.

May fireform a case from the 7mmRM to clarify the case length since chamber casting does not give you the base end.

Thanks again!!

Michael