Which model Series One is it ? Is it an ex-military one? I am by no means an expert but my first guess would be that it has a replacement engine , possibly a military one . Is it a 2ltr engine , or maybe a 2 1/4ltr .?
I would be very surprised if you did not get an answer on the Land Rover Series One Club Forum , so that ought to be your next port of call . There are not many Series One owners here , they are mostly modern Land Rover owners , and are as such pretty ignorant on the subject of proper Land Rovers !