There's also a Colnago-specific user group on Facebook that another forumite recently mentioned to me. I would say that this forum and eBay would be two of the best places to list the bikes. 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.
