1946 I think, I have a BJP Almanac with the details back in the UK.
Originally Posted by Marco B
That was the point where the Internatioanl backs became standard and the Imperial & Metric size holders would fit the same backs.
Prior to WWII there was a major problem in Europe where there were no agreed standards and plate (film) holders and even roll film backs weren't interchangeable between all cameras of the same format. For historical reasons Zeiss made 3 variations at one point.
So after 1946 a European 9x12 DDS (double dark-slide) would fit a 5x4 camera and vice versa, and similarly with larger formats.
The flat sided DDS that we use now was used as far back as the 1890's but it seems to have become more Universal in the US earlier than the UK but backs became more standard with the introduction of film packs and roll film holders. Some of the early Kodak's made in the US & UK would take modern film holders.