Roger:
The Field side-mount is commonly found on early Lee Enfields, but not sure if "one size fits all" or whether it is necessary to get a specific model to fit your rifle.

Also, won't work with a dust-cover.

There's only one on UsedGuns at the moment, came off a Remington 33.

Here is a pic of the Parker Hale version, posted over on Gunboards:

I have a Field side-mount on a Sportco No.4 Mk.I .308, set up like an L42A1 'sporter'. Scope is a Simmons Whitetail (for now, anyway) and with the long, heavy barrel that outfit shoots very well indeed.