you may be able to guess the first four, if its a '54 or '55 86 the first number should be 4 or 5 second number 7 (Land rover), third number 1 (86 inch), if yours is made for the home market the 4th number should be 0.
then the sequence changed for 1956 on but the table explains it
if nothing else it should show how much of the number is missing.