Tough one to dig up, but with the high workload in a non-augmented tandem, it seems reasonable that those would be the designers that first had a reason to pursue augmented helicopter flight.
Bit of research and I found that the HUP prototype first developed in 1948 flew with a Sperry Autopilot to develop the HUP2 without the large rear stabilizers. I couldn't find the date on that test, but the paper, "Piasecki Test Autopilot System" was released some time in 1951. In June 1951 Meyers, Donald N. and Vanderlip, Edward G. published “Helicopter Stability via Automatic Control,” in Aero Digest, vol. 62, no. 6.
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SASless the SAS control switch in the H46 I flew was identical and very near the windshield wiper knob. One IP would occassionally play the uncooperative FO. Flying in moderate rain, I asked him to turn on the wipers, instead he turned off the SAS. Definitely recommend doing this only to experienced pilots, I could handle the SAS Off well enough, but learned a lesson in supervision.
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Of course, the horror stories of non-augmented flight in a tandem mostly apply to new pilots. Transit, IF flight, rescue sequences, etc are all possible albeit with a very high workload (Cooper-Harper of 8 or 9).