85lc:
Its not a simple process, and you should search these forums for the current status of the records and protocol for researching your Jeffery rifles. Pretty sure its been discussed/updated in the last few years.
I went through Paul Roberts in the UK years ago, when he was collating requests and forwarding them to Thomas Friedkin in the US, who owned the Jeffery name, maybe twice a year iirc.

Note that the sale register no longer exists, only the workshop records. Can't remember the reason but maybe tossed out during a cleanup following the London Blitz?