The Breitling Watch Source Forums

Hello,
I'm new to the forum and wanted to start out with a question regarding a Patek Philippe Chronograph that a friend of mine has. The watch I'm refering to appears to be a triple date moon phase calander model in 18K pink gold. The band is brown leather with a gold deployment clasp. The crystal is sapphire. It also has a sapphire crystal back cover. My friend mentioned that this watch was made sometime in the 70's or 80's. I don't remember ever seeing a Patek Philippe chronograph with the clear back from that era. I'm wondering if this could be an aftermarket addition? I'm no expert, but would like to ask the experts. I have a number of my own watches which I intend to post and discuss on this site. Thank you in advance for your responses.
Mike