Maybe not much help, but my best guess on this one would be that at some point, the filter was adapted to an aftermarket replacement filter that is more standard and easily obtained. My first suggestion is to look for any numbering on the old filter that might indicate manufacture and manufacturer's number designation and cross reference to a more commonly available filter.
Failing that, I would remove the old filter, drain it, and take it to a parts store to match up. There are a many sizes and shapes of filters available. You can start by matching the thread size/pitch and gasket diameter. Better suppliers may be able to find a suitable replacement based on dimensions.
Hope this can be of some help to you.