It's always a relief to see the solution to something which has puzzled one.

The current UK vehicle registration number allocation system uses two digits to define the point in time in which the vehicle was first registered, thus 01 means the first registration period of 2001 and 51 the second.  I've often (well occasionally) wondered how they would cope with 2011, whose first period would clearly be 11 but whose second could not be 51 without confusion with the second period of 2001.

I need not have worried.  Indeed if I had only googled UK vehicle registration number system when first the problem entered my mind I would have found that the 61 I saw on a car the other day was a consequence of  the basic principle behind the system, which is to add 50 to the year digits for the second period.

Thus I would have been able to sleep soundly long ago, except that I'm now wondering what will happen in 2051.  Alas I may never know.