iMac g3 serial number invalid

Were you able to check with several online identify-by-serial number sites?

Is the number you have (in the computer hardware, or in software) one that

may have been given to a factory refurbished computer or a replacement

logic board that may have been repaired? Those numbers can be flashed

or in some cases replaced by a different number. Apple official refurbished

models usually have a different prefix letter code than their new counterpart.

With everymac database you may also use Sales order number, model number

and other on-site recognition of hardware to identify a specific model in hand.

With a model number, you may be able to find information about the series

build in general, as with http://everymac.com site; or powerbookmedic here:

https://www.powerbookmedic.com/identify-mac-serial.php

A different lookup here: http://www.everymac.com/ultimate-mac-lookup/

another less detailed site offers info by serial number also, but may reject it too:

http://www.chipmunk.nl/klantenservice/applemodel.html

Do you need to use the serial number for locating replacement parts?

There should be numbers on the hardware, directly on the logic board;

sometimes these are different, later replacement parts were offered to

cover more than one model. And third party remans could have different

number series, or were flashed and erased, instead of a substitute/new.

Hard to say why the serial number you have presents an invalid response.

In any event...

Good luck & happy computing! 🙂

What do you mean by a "Rev. B" slot-loading iMac? Unless you have a tray-loading iMac from 1998, that designation doesn't apply to the slot-loading models. Did you research the serial number found on the sticker on the underside of the housing?