The //e Card is able to replace the standard Apple II beep with a Macintosh sound. The means by which this is accomplished is quite interesting.

The routine at $fbdd (BELL1) is

  .org $fbdd
  ; this code replaces the .1 second delay
  .byte $02  ; two byte NOP in 65C02
  .byte $01  ; should be ignored by CPU
  rts
  nop
  nop
  ; remaining code matches the original //e code and calling $fbe2
  ; will produce a beep, at the wrong pitch if the card is in fast mode.

$02 is a two-byte NOP on the 65C02. Interestingly enough, when the processor on the Card executes the sequence $02 $01, it produces the configured beep sound. So far I have found no effects for $02 $xx where xx != 01.

Try this in the monitor:

*300:02 01 02 01 02 01 60
*300G

Three beep sounds!

So this must be part of the magic that interfaces the card to the host Macintosh. How very very interesting.