Marky,
The O18 was 73 to early 78, the serial number was 18,000 so that made it towards the end of production so it was an F and not C, it could have been an X but they are very rare. It could have been a 2, rather than a 1M, but without pictures I went for a 1 as there are very few with 2 in the prefix. And the serial number being 18259, I found 18251 on the Australian Moke Register and it is July 1977, so that gave me the month/year.
O30 is is 'Export' Moke built in the first three months of 1973 and the serial numbers never got higher than 2000, so it was not one of them.
Terry