Pityt that Pape did not have any records, worth a try anyway.

The company of Charles Lancaster Gunmakers holds some Lancaster records although they are incomplete. They used to have a web site but not anymore. They used to produce a letter or 'Authentication Certificate', which there was a fee for.
The company was/is owned by Ronald Wharton, the last e-mail address I had is wharton@bunduki.co.uk
If they have a record for this rifle the Lancaster number of A700 should make it easy to find, in the ledger books.

Regards

Alan