The Shimano 105 groups cost less than the Ultegra 600 group during the 1987 to 1989 period. But, it may have been Shimano's best group. Shimano introduced indexing in 1985-86 with Dura-Ace, and it worked reasonably well. Then indexing was put on Ultegra 600 in 1986-87, and it worked even better.

But, around 1987-88, all the "kinks" had been worked out, and the "improved" indexing was put on the 105 group. That group also got Shimano's best ever brakes, the SLR system. The 105 SLR brakes of around 1988 were so good, some folks replaced their 1986 era Dura-Ace brakes with the 1988 105 brakes. Finally, around 1989, Dura Ace was "upgraded" to shift and brake as well as the 1988 Shimano 105 group.

So, if your Centurion has a 105 group, you have a terrific bike. I have owned Centurions with both Ultegra and the 105 groups, and liked both. But, the 105 was the better of the two.