So, perhaps that is where the serial numbers are on your bikes. However, I looked at an older Master that I have from the same time period as your C40s and it has the serial number on the drive-side rear drop-out and not on the derailleur tab. It's on the derailleur tab just in front of the rear pop rivets on the tab (behind the derailleur).
There definitely isn't a serial number on either rear dropout. I just looked at my bike again and took the rear wheel off. But I searched all over the frame w/a flashlight many times and found nothing not hidden by rims, etc. However, just to be totally honest about how dumb I am, I had to Google front derailleur tab and look at pics to know what you're talking about, so may be looking in wrong place. There's a warning label there that says to read the manual before operating but I don't see any serial number.
