Bally dropped the Feature build-up score reel at some point during the Turf King production run. They issued a new schematic (XW-319c) for games without the Feature build-up score reel. The later games have serial numbers starting with AB. My game has an early serial number of A2900. Both schematics are available on ipdb. I'm not sure if any other changes were made for the "AB" Turf Kings.
I've got an idea why Bally dropped the Feature build-up score reel. The early Turf King ("A" serial numbers) have two counters in the front cabinet. One counter is for the traditional score reels, and it ticks up by 1 for every 1 game that is knocked off the 3-digit score reels, just like a bingo. There is a second counter for the Feature build-up score reel. It is incremented when the Feature payout score drum is incremented, so that counter has a different set of rules. The accounting/bookeeping for the Feature build-up required the operator to subtract the number on the Feature score reel on the backglass from the number on the Feature counter in the front cabinet, and then compare that result with the number from their previous collection. So it's a hassle to manage the two counters. Worse, the reset (knock-off) button only works when the Feature flag is lit and a ball is in the Feature hole. So imagine you're the location owner paying off a maximum $45 winner, and another player drops a nickel before you get back to hit the reset button you forgot about. The reset button won't reset and the operator won't know that you paid the $45 winner (nearly $500 in 2018 dollars). Sounds like a recipe to have the operator and location owner at each other's throats. My game came with 815 on its functioning Feature counter, so hits were rare and probably high value to boot.
Turf King could be configured through switches to pay 320 on the regular score reels for a Feature hole winner, so it's not obvious that that a variable payout of up to 900 ($45) would drive much incremental business. I'm guessing Bally got a lot of operator blowback and dropped the Feature build-up. I believe the Feature build-up was always present on Grandstand, the payout version of Turf King.