Hi Mick,
From your description it sounds as if your engine has been fitted with pistons for a Honda CB250N. Now, the genuine Honda pistons have the 3 piece oil ring whereas pattern pistons used a one piece ring. I have just checked the pistons I have, and both the original and pattern pistons have the same drillings for oil drainage and the same width groove for the oil ring - so there is no reason why you can't fit the three piece rings on your pistons. If you would prefer to use the one piece ring, you could try contacting David Silvers (Honds spares) - he used to stock genuine and pattern parts. (Sorry, I have mislaid the contact details, but he has a website).
Regards,
Eddie.