What does it mean? What is it?

[D. Kania]

So, up front I am not really a collector I've been gathering PYREX, CORNING WARE, FIRE KING at yard sales, flea markets, charitable stores, etc for about 4 years to resell. Have found mostly common stuff that I'll loose money on or break even at best . Have found a few hard to find items and very few that I can not identify at all other than the model numbers on the item. I'm not here to sell to any of you. I'm seeking information that I don't seem to be able to look up on the web. Maybe, some of you with your expertise can shine some light on my conundrum. I just didn't know where to go for the answer.

Example: PYREX LID P 81 C, have 2. On the opposite side of the model number is a 12 on one and 24 on the other What does the 12 and 24 indicate? I did notice that they weight differently and have minimal differences in dimensions.
Example: PYREX 622-C B-U clear glass lid. What might it be for?

I'd appreciate any help you might be able to provide and apologize if my inquiry offends or does not meet with the standards or etiquette of your forum. Thank you.
Doug K.

[Doug D.]

P81C is a Corning Ware lid. The 12 and 24 would just be mold numbers. A 622-C is a knob top lid for a 1 quart Pyrex 022 round casserole. The B and U must also be some kind of mold or production line identifiers. Lid model numbers for many Pyrex pieces can be found on this page: http://www.pyrexcollector.com/models.php