OK. So I filed the I-751 10 days before GC expired(12/20). The info line gave me the receipt number and tells me I should receive NOA soon to extend status - they may have even told me it has been sent. BTW, online case status does not recognize receipt number. Does this mean that I am not authorized to work until I receive the NOA, or since I know they have begun processing, am I ok? What do I do?

You are authorized to work. (However, you might have trouble if you needed to get a new job, because you are required to prove your authorization within three days of being hired.)

The online case status will not recognize your first receipt number. You may get a second case number later that it will recognize.

Thanks for setting my mind at ease. Nice to hear from a fellow US/Scot couple!