I believe the SSW unit was the first B36-7, and was very much an experimental/test/demonstrator unit: I think the idea was to try out GE's last technology, which would have included Sentry and, I assume, the GTA 24. But I'm not sure. The "package" with the GTA 24 would also, I think, have included the uprated (with finer-toothed gears) 752G traction motors, so if you have a source that will tell you what the unit had as motors, that might be an indication.
Santa Fe's were the next. Somewhere, when they were new, I recall reading a remark by some trackside observer, suggesting that they (and the more or less contemporary GP50 units from that other builder) showed noticeable improvements in performance (particularly what they could pull climbing hills): again, suggesting the new technology.
So my GUESS would be that your initial impression was right, and that all US-built B56-7 (and C36-7) had the new generator and Sentry.
(There was one smallish order for C36-7 from NdeM early in the Dash-7 era: the units were built by GE's Brazilian subsidiary/branch. Those units may have been old-tech: I vaguely recall seeing somewhere the comment that the 36-7 line didn't really catch on until later.)
(There is also the 30-7A line: 3,000 hp from an FDL-12. I think the first big order was from Missouri Pacific for 50 B30-7A, and I think they had the new (Sentry) wheel-slip feature, which I assume involves the GT-24. MP had gotten three B23-7 units before that which were up-rated to 3,000hp, and may NOT have had the new system. On the other hand, Conrail's 50 C30-7A of 1984 seem to have had the GTA-11. Go figure.)
Apparently the GTA-24 was structurally similar enough to the GTA-11 that many GTA-24 were created by modifying GTA-11 generators. Which, given human beings, could I suppose led to some errors in record keeping...
