The EN temperature ratings were only invented some time ago (2002 is the earliest I found on a quick web search).. they did not exist for the older bags ... so your out of luck with older bags.
If the older bags existed in the past on the inter net then the web archive might get you something on them.. but you'll need some sluing to find them still ...
https://web.archive.org/Other than that your looking for people who have old catalogs in their stored stuff.
What you can do is - weigh it - the weight can indicate its age (older materials are heavier) and measure its loft - remembering that some will have lost loft through use, age and confinement.
I have a few older bags myself .. I think the oldest is a Paddy Palin Kiandra ... 1970s?
These bags can make usefull base (car) camping bags ... they may need refurbishment with fresh insulation to regain their performance.