Re: Old HI-Lift Jack
It should have a rating and you should be able to find and identify the shear pin (meaning it hasn't been replaced with a bolt...)
"Modern" Hi-Lift jacks are rated for 7,000 lb and the running gear is cast instead of bent and welded.
I've been to the Hi-Lift HQ in Indiana, they're good people and will surely provide more information. Regardless of what you have I would seriously consider spending the ~$70 to get a modern Hi-Lift and save that one for display or sell it.