It means you have a piece of silverplate that has 12 pennyweight of silver plated onto a base metal.
.....91 pennyweight worth of what??
A pennyweight is one-twentyith of a troy ounce. Troy ounces are used for precious metals such as gold, silver and platinum.
One pennyweight = 1.56 grams
The pennyweight was the weight of a silver penny in medieval England. When pennies were introduced in England in the 8th century, their original weight is believed to have been 24 grains. 24 grains = 1 pennyweight so 44 pennyweight = 68.427649 grams.
The pennyweight would be worth 55.00
12 dwt is 12 'pennyweight'. A pennyweight is a measure of weight equal to 7.586 carats. 12dwt would be 12 x 7.586 = 91.03 carats or 0.6 troy ounce.
pennyweight
Pennyweight. In the Middle Ages, a British penny's weight was 1⁄20 of an ounce or 1⁄240 of a pound of sterling silver.
One pennyweight is exactly 1/20th of a troy ounce or about 1.555 grams. Pennyweights are obsolete today.
Around 3.5 Pennyweight 1.56 grams to 1 pennyweight
I have been told that the 12 denotes the DWT or pennyweight of the item. It is supposed to be silver and not silverplate. I am not sure what pattern you have but check it to see if it is silver and not silverplate. I think the value can be determined by finding the actual pattern name, or if you want to scrap it, see what silver is bringing per ounce right now.