Jimbo First class clams are generally purple/dark purple blue, perhaps with black strips, purple striped, very basic looking.
Ultra clams, are the colored clams, turquoise, green, etc, etc, with intricate patterns and designs. There is a dramatic difference between them.
Many vendors, including some sponsers on this site, label 1st grade clams as ultras. It's not really a marketing term, although at the same time it IS a marketing term. Afterall, what freaking label in this society is not a form of marketing.