Hello John,
As Steve said, it is very odd indeed not to have some form of ID on a bearing. Just buy yourself a cheap set of vernier calipers (any modeler should have one anyway!). all you need do is measure the outside diameter, inside diameter and bearing thickness (make sure that if the answer looks strange then you probably have a metric bearing and imperial calipers or vice versa).
That is all the info you need to find a replacement bearing, simple as that. Just go on any bearing manufacturers website and you will find what you are looking for. The various grades of bearing are not important in this particular application. Just get the cheapest one you can (but don't buy Chinese bearings!).
I have a couple of very late Merco 61s which are the target drone ones if I have my facts straight(?). I am tempted to strip one down and measure the back bearings and compare them to the standard ones. I really cannot see why the manufacturer would do a thing like change the bearing size at such a late stage in the life of the engine.
Regards,
Andrew.