Hi Greg
The SP2/3/4 and 5 were all 888 cc. If it was an SP2 (1990), the chassis number would confirm this, ie; ZDM888S*****. The 851Strada was numbered ***851S****.
The 888cc SP2 did have low mounted mufflers, but it came (as did all SP models) with an aluminium subframe beneath the seat, and this wouldn't support a dual-seat. It also had Ohlins suspension front and rear, and a silver plaque on the top yoke bearing its production number.
Maybe Phil911 has hit on the right answer to the 'TC' initials. (The only other 'TC' 851 was the early 'Tri-Colore' - but I don't know if this would be recorded on the docs).
Steve