Thanks for posting. This looks like a 15-228 Pivot Shoe. This shoe uses either 3/8" thick Series 165 Coil Balances, or the 1/2" thick Series 160 springs. You'll need to remove the existing assembly and measure the thickness of the springs to confirm which Series you need.
The parts for this particular setup are all sold separately. For each individual coil spring, you have to choose a spring weight, which denotes the weight the individual spring can support. I wouldn't necessarily go by the stamp on the housing for this setup--instead, look for stamps on the springs themselves. If there are none or you're unsure, give me the exact weight of the window sash these springs support, and I can work out what you need.
That's uncommonly heavy for a window that uses coil balances, but still doable. Our Series 165 spring only carries 8 pounds per spring at maximum, so if you still use 3 springs per side that's only giving you 48 pounds of total support. That's a little too light for my tastes so I'd advise using 4 springs per side instead of 3.
The Series 160, being larger at 1/2", tends to be stronger. You can use two 9 pound springs and then one 7.5 pound spring on each side. That gives you 25.5 pounds of support per side, which is perfectly sufficient for a 50 pound sash.
It will ultimately depend on the size of the spring. That should be the next detail you check.
