I was wondering the same thing Sparky. I have seen many 100amp Meter pans with one (central) lug. It must have been permitted at one time. Our meterpans on LI even have a separate lug for a bonding wire if necessary.
Bill, When I first started in the trade, the local utility supplied all of the meter cans and they only had one neutral conductor lug, but their rules required that the neutral be continuous from the weatherhead to the service panel, so only one lug was required. Don(resqcapt19)