During the Swan350 & 500 era (1964-67) Swan began struggling with the growing drift problem. Dozens of areas were examined and improved upon. One was the VFO board itself. For the first run of 500C's and the model 410C external VFO's an extra buffer stage (Q3) was added to further isolate the oscillator (Q1) from outside influence. It was later determined that the original board circuitry was not to blame for any significant drift other than normal warm-up; so, the 3 transistor design was dropped from both the transceiver and external VFO. Although the failure rate on Swan VFO's is extremely low, the 3 transistor unit failures are slightly higher than the 2 transistor units. Yes, they are interchangeable. - - K4BOV
