Steve, I have noticed at least two UK sellers on ebay who quite obviously strip the lenses off 9x12cm cameras and sell them separately, a good the lens on it's own sells for far more than a complete 9x12 camera, in the UK.
Somewhere I have an image of a non-Voightlander 9x12 which took interchangeable lenses, with a bayonet mount. My guess though is if yours is a Bergheil someone sold the original lens for 5x4 use. My Rodenstock camera with an excellent Eurynar cost me around £30, currently 2 Eurynars are on Ebay for £120 +£125.
Ian