Window channell

Based on your photos, you have either our Series 380 or Series 385. The only difference between them is the size of the bottom fitting, which is indicated on their respective store pages. You'll need to measure your bottom fitting to confirm which one is the better match.

Since there is no stamp, you need to weigh your sash on a scale to confirm the best option to go with.

Comparison of the S378, S380 and S385 balance shoes.