T-Mar

I'd say it's March 1981. Sometimes Bianchi used two characters for the year but it was generally only one. Based on the frame chracteristics, yours in clearly not 1978 or 1988, so the '3" would appear to be the year indicator. The Ebay bicycle exhibits the chromed caps and fork crown that I would have expected to see on your frame.

Edit: The rear derailleur will a date/patent code on the upper housing, next to the cable housing recess.