Hi Pat,
Unfortunately the Law labels do not offer much to identify the mattress except for it's size(4/6 is the full size). The word Somerset may be a clue but the brand is not mentioned so it is like searching a for a needle in a haystack.
Do you have anything from the time of original sale or are there any other indicators of brand on the bed? On some major S brand beds, they usually have a satin ID label of some kind sewn to the mattress as well giving the name and comfort level. WIth out that info, the best you can do to match it up is to find something equally as thick and try a few options to see if any feel similar in firmness/softness.
Andrew