73 Midget Rheostat

["Tyson Sherman" <tsherman@tecinfo.com>]

Upon removing the dash from my 73 Midget I noticed
that it has what appears to be a dimmer knob.  It
looks to have been added, and my 72 Midget lacks
the knob. 
The control behind the dash looks just like what the
Bentley manual calls the rheostat (for 75-80 MGB).
However the knob looks nothing like that for the
dimmer in my 76.
So my question: Is this original, optional, or added,
and if it was added, does anyone know when?
Thanks in advance!
Tyson Sherman
<http://www.tecinfo.com/~tsherman/>