If you have London proof house number & date stamp on the barrel, their archives if they still have the info. should be able to show to which retailer the rifle was returned.
The retailer would order the rifle from Ges. Steyr and when it arrived, they would send it to the proof house for proof. The rifle would then be returned to the retailer for sale/supply to their customer.

These days, no proof in the UK is now required as the Austrian/German proof is fully recognised, unlike USA manufactured weapons which must be proofed in the UK prior to supply to customer.