This is a very nice vintage Rolex Air-King reference 5500 with calibre 1520 from the late 1970’s that I recently serviced for a ClockSavant customer. The Rolex Calibre 1520 does not use the Micro-Stella balance wheel regulation system found on many other Rolex calibre’s. Instead it uses a more traditional regulator on the balance cock along with a screw-less balance and two “inertia blocks” positioned on both sides of the balance wheel arms. The idea is that you do major regulation with the balance cock regulator and fine adjustment with the inertia blocks. In-practice, if someone starts adjusting the inertia blocks and fails to do it with absolute symmetrically, the balance is effectively thrown out-of-poise and it can be time-consuming to try to bring the watch back into regulation. In the second picture showing the two parts on top of a blue material (the blue substance is a play-doh like material watchmakers use called rub-off), the part to the right is the winding pinion. This part was worn and benefited from replacement. From a servicing quality and methodology standpoint, if you don’t fully disassemble the watch, you won’t see this wear. It still worked correctly but likely only had a few years of regular use left before it started to fail and damage other parts along with it. Videos showing more detail relating to this post are available on the ClockSavant Instagram page.