It appears that the side Pivot Lock Shoes for the top sash of one of our sun room windows have long since been broken. I was able to fix the left shoe, but the right shoe is missing the metal piece in the middle, hence I had to order a replacement. In the meantime, the sash is supported by two long wooden planks that fit perfectly to the sides under the sash.
I went to Home Depot, thinking I could find the shoe or at least some spare parts, but some $3.xx "U" shaped metal piece but didn't work as well as I hoped. I gave up. Home Depot said that there are too many types of window sashes and shoes, therefore they do not carry the shoes, and that I would have to call the manufacturer directly.
I found the best matching parts on Ebay and Amazon.
The replacement pivot lock shoe should be this type: CR LAURENCE 62565 CRL Pivot Lock Shoe: Measurements: 17/32" (H) X 1-1/4" (W) X 2-1/16" (L)
The above is the closest match to our type of widows.