you can tell what gauge it is by looking along the seams. It looks like a fairly sturdy gauge there, you could tell by poking a pocket screwdriver into a seam and twist a bit to see how tough or thick it is. 4-5 hundred pounds could right for certain gauge, but from what can be seen it might be a little heavier gauge sheet than a newer one would be. Depending what part of the 50s it could still be leftovers from WW2 or still manufactured the same. If it was closer to, or into the 60s, that's about when quality began fading, and cheap began taking over