⭕ Two-letter ES paint code meaning Entered Special for a special order or a show guitar. This was used by the factory to tell the finish booth to paint this body special order.

And this from Vintage Guitars Info: I have also seen the ES stamp on some 1966 sunburst instruments!

This was my uncles guitar - He was the third owner. This guitar has been in its case for many years. This guitar plays like an old friend. Stays in tune and rings forever. Classic Fender Stratocaster sound. This guitar has been babied. Neck straight as an arrow. This guitar has no issues other than slight buckle rash and a ding here and there - this guitar is in excellent condition. The finish is truly amazing with tobacco browns and vivid reds and beautiful yellows. Thin nitro finish must be seen in person to fully appreciate. Fender did pay special attention to this one for sure. Original case (one latch doesnt spring open but still works), whammy bar, tremolo cover and rear plastic guard included.

1966 and the ES Stamp.
In 1966, Fender used the ES code a lot on their custom color instruments. At least for 1966, the ES code was used as some sort of default for custom colored instruments (be it Teles, or Strats or Jazz Basses). This two letters (ES for Enter Special) seems to denote a special order, at least for 1966. Again this is has been seen lots of times on 1966 documented original custom color instruments. I have also seen the ES stamp on some 1966 sunburst instruments! (Perhaps these guitars were special show models, so extra care was taken in their finish.) The 1/2 tall 2-letter code were a factory color code, and are completely different than the 1/2 tall three-digit codes. The two-letter code appears to be CBS/Fenders way of specifying bodies to be painted a custom color for a special order once the body was in the spray booth. If a guitar has a two letter code on the body *only* (and not the neck), again this confirms the 2-letter code was an original factory color code applied just for particular special orders (like a Fender NAMM show guitar or a special dealer/artist order).