There's an easter egg hidden in the Super Graphix Gold printer interface for Commodore computers. In fact, there's a few of them, but it's a bit of a journey to see them, mostly due to printers.

The original Gold egg: https://eeggs.com/items/30605.html
HD6303 Instruction Set: https://www.jaapsch.net/psion/mcmnemal.htm  
Psion info: https://www.jaapsch.net/psion/index.htm
Super Graphix Gold v1.2 ROM: http://8bitshowandtell.com/downloads/SuperGraphixGoldV1.2-TMS27C256.bin.zip

Index:
0:00 A look at the Super Graphix Gold
1:50 Interface options
3:50 Searching my basement for a Centronics-compatible printer
6:11 Laserjet 2300 - nope
8:09 Panasonic KX-P1124 - yep
9:39: 9V DC 200mA Center-positive supply for the interface
11:24 The WD-40 "Rejuvenate" Trick
13:06 Hello World
14:23 The Easter Egg: "+-"
15:46 Looking inside the interface
18:41 More eggs: "%&" and "FLICKINGER"
21:18 The manual is great
22:20 Thanks to my patrons