Gas lift struts for trailer question

I have an old (1972?)semi-clamshell type trailer. When open, 4 gas struts hold the "lid" up. Two look like original equipment from Germany and two from an, apparently defunct, company (Colmar Gas Springs) in Pennsylvania. In cold weather the hatch won't stay open. Any idea as to how to determine the lifting force necessary for new struts? I didn't see that number on the existing struts but it may be there. I was able to get the P/N off the American struts, thought that's all I would need and stopped looking. I found out today they were a two person company and the phone number doesn't work. The trailer is now 300 miles away in winter storage so I don't have ready access to look for more numbers. No rush, the spring ridge season is a long way off.