This screw thread mount SMC Takumar 200mm f/4.0 was purchased in 1975 to use with a Pentax SP500 camera. I continued to used the excellent lens after acquiring KX and MX cameras. But mounting it on the K mount cameras with the required Mount Adaptor K was cumbersome. So, I decided to permanently convert it to a bayonet mount. First I determined where the lens locking pin of the camera would be when the lens was correctly mounted. Using a small metal file (about 3mm in width) I made a notch for the lens locking pin. The next step was the removal of the restraining spring from the adaptor. Before screwing the adaptor on the lens, I applied a light coat of silcon cement to the threads to secure the adaptor. (NOTE: Most recent articles recommend not using any cement.) The conversion continues to work well after 25 years, even with the digital Pentax *istD. Not bad for a lens designed in the era of the slide rule.