The Stovebolt Site Forums The Shop Area Engine Shop 216, 235, or ???

Here is a good description of the differences through the years:
Chevy Six features
Have a look at picture # 13. The mention of the "smooth" contour is a key point. If you have the smooth contour, it is a 50 to 52 low pressure. If you don't but it has the triangular plate, it would be a 1953 low pressure motor.