30 inch single hung window spring mechanism with stamp code 2940. Window mfg Lawson Industries

Hello, thanks for posting! It looks like you could have either our S380-30 or S385-30 with the 2940 stamp option. The difference between the Series 380 and the Series 385 lies in the depth of the shoe. Take a look at the picture below for more detail. Once you have determined this dimension you should be able to select the appropriate balance. 

Comparison of the S380 and S385 balance shoes.

I don't believe we have an exact match for your top sash guide. However, take a look at the list I've created on the side of the page. Review their dimensions in comparison to your original part to see if any of them might work as a substitute. The crucial dimension is the width!