If you are registering Ryeland tups, they must be registered with a name starting with the correct year letter - for example tups born in 2012 will all be Sam, Superman and so on. Ewes don't have registered names, just a number and the number (for ewes and tups) doesn't include a letter. The importance of the letter ony relates to the name of the tup. (Although our three ewe lambs from last year are Stella, Smudge and Sweatpea - these are only pet names).
The numbers on the tags have nothing to do with the letter, so you can use them.