Given the information in this thread I would probally come to the conclusion that they are 500N type when you divide the approx 120 lift to be 54.5 KG you would also have to calculate overall length and circumference and know the variables. Temperature will always affect the lift pressure.
(You wont need to do the calculations and pay for custom building)
http://www.car-stuff.com/mmparts/hum...unk_strut.html