Thank you very much for your answer! In addition to your information that I learned on the official website of the Birmingham Assay Office: PRE 1950S EXEMPTION
Articles which should have been hallmarked when they were made, but bear no hallmark, are now treated as exempt if they were manufactured before a specific date. Since 1999, the date has been 1920, but the amended legislation alters this date to 1950. Therefore, any pre-1950 item may now be described and sold as precious metal, if the seller can prove that it is of minimum fineness and was manufactured before 1950.
6th April 2007 also sees another amendment to Hallmarking legislation in respect of items brought onto the market pre 1950.
Prior to this date it was not compulsory to hallmark all precious metal articles, and up until now unhallmarked items manufactured after 1920 could not legally be described as silver, gold or platinum.
The new amendment extends the exemption date to 1950 and allows these items to now be sold as gold, silver or platinum without a hallmark, so long as the seller can prove the fineness of the precious metal and that the item was manufactured before 1950.