Actually, the 'window cap' is a rear window track guide for the rear of the side window, that installs at the upper rear at the back of the door, and they are different (early vs. late) in their application.
This particular part# appears to be for the 'later' doors, but I don't remember the exact year of the change ... (but I think '75).
Mark's site has repros, but there's a typo in the description, showing both sides as part # 914.542.173.10 (which is the left side #). I'll send him a PM alerting him to the fact
https://shop.914rubber.com/914-Late-door-en...ght-914LDEC.htm