Uberstationmeister and Railking are on the right track.
QSI reverse units and subsequent sound systems were designed for Lionel postwar transformers. Around 1990, when new electronic transformers came along, QSI units could "read" their waveforms incorrectly and malfunction. This is the first time I have heard of a sticker. It probably addressed that situation, as RailKing posted. I think MTH corrected the problem in later production runs of transformers. The crowning achievement was the Z4000 with pure sine wave and electronic programming of RESET Features in QS-1.
Back in October, about a week before the York Meet, I purchased MTH Premier Southern Ps-4 No. 1401 (20-3006-1) from a Forumite. Made in 1994, she was among the first to have QSI "ProtoSounds." This elegant Georgian locomotive ran with my postwar ZW but not with my MRC Pure Power. I had a hunch that might happen. MTH Parts brought a board and a chip from a Railking Ps-4 (30-1125-1) made in 1997. Wayne Renga of J & W Electronics installed them. Now my Ps-4 runs with other transformers, and I gained Passenger Station Announcements (PSA).
I took a photo of the original circuit boards in the tender. I'll post it here. The only programmable feature is (6), Chuff Volume.