This is from Tractortips.com:
Serial: 100001 | Year: 1991 | Plate Location: REAR OF TRANS CASE BELOW PTO
Serial: 1001 | Year: 1989 | Plate Location: REAR OF TRANS CASE BELOW PTO
Serial: 115001 | Year: 1992 | Plate Location: REAR OF TRANS CASE BELOW PTO
Serial: 120001 | Year: 1993 | Plate Location: REAR OF TRANS CASE BELOW PTO
Serial: 130001 | Year: 1994 | Plate Location: REAR OF TRANS CASE BELOW PTO
Serial: 140001 | Year: 1995 | Plate Location: REAR OF TRANS CASE BELOW PTO
Serial: 150001 | Year: 1996 | Plate Location: REAR OF TRANS CASE BELOW PTO
Serial: 4111 | Year: 1990 | Plate Location: REAR OF TRANS CASE BELOW PTO
It looks like that 770 was built in the later part of 1989 ('89 SN's start at 1001, '90 SN's start at 4111).
I believe this is the year you thought it was in an earlier post.