ECHO-sound synthesizer cards

I got few pairs of ECHO Sound Synthesizer -card speakers awhile ago and started to look for the interface card as well as one did not come with them.

Luckily i was donated pair of cards  (thanks Will!)

echo_speakers
“ECHO”-speakers. Is there reason for different color of the button?

echoplus_echoiib_cards_small
-Echo 2+ (have mono and stereo outputs)
-Echo ][b

echoplus
ECHO+ installed to my “main” Apple //e platinum.

Video of the “Alien Clonepure” & ECHO+ sounds:
https://www.youtube.com/watch?v=bW4lYkC3-KY

Wikipedia:
https://en.wikipedia.org/wiki/Echo_2

Manual:
ftp://ftp.apple.asimov.net/pub/apple_II/documentation/applications/misc/ECHO%20II%20Synthesizer.pdf

apple_appleii_echoii_board
Found this image from the internets, so this is how the retail package have looked like.