Shimano 600 Ultegra was released in 1988 but the dual pivot brake option wasn't available until 1991. The STI brifters were released in 1992, so the presence of down shifters suggest it's 1991, provided everything is OEM. The red bordered fork decal, if OEM, indicates SLX or TSX, as opposed to SL. The bicycle obviously needs an overhaul so removing the bottom bracket will allow you to inspect the inside of the ends of tubes to determine if it actually is SLX or TSX.