constant force balance - Replacement parts

I agree, the pivot shoe looks like our 15-191, along with the 15-150S support. As far as the Coil Balance goes, don't worry about the overall appearance--the Series 160 is compatible with this pivot shoe. However, so is the Series 165. To know which you need, you must measure the thickness of the coil tape. If it's 1/2" thick, then the Series 160 is correct. If it's 3/8" though, you need the Series 165. 

As far as the right spring weight to select, go by the stamp codes on the spring itself. If there are none or you are uncertain, give me the exact weight of your window sash and I can work it out for you.

I would double check that exact sash weight to be safe, since it seems pretty light, but if so, then using (1) of the S160-3 3.5 pound springs on either side of the sash should work well. That will equal 7 pounds of support total--and yes, 1/2" wide springs mean you need the Series 160. 

Yes. If you went with the S160-5 5.5 pound springs, that would equal 11 pounds of overall support. This is significantly higher than the weight of your 7 pound sash, and would result in the window being very difficult to close or keep closed. With window balances, you want the weight range to be as close as possible to the actual weight; with coil balances specifically, being within 1 pound is best.