According to the Owner's Manual, the UT-121 Digital Unit is Already Installed into the IC-91AD. It is optional for the IC-91A. Current price for a new one is $229.95 at HRO. The Owner's Manual recommends having the dealer or a service center install this option.
I purchased a 91A when they first came out and later purchased and installed
the optional digital board. If you purchased the 91AD the board was included
and already pre-installed.
That board was not easy to install as the contacts on it are soooo tiny i had
to use a magnifying glass to be able to see if it was correctly lined up before
pushing it in the socket.
The 91A/91AD is a great little amateur radio ht. If you are contemplating a
purchase of one i highly recommend it. Also serves pretty well at being a
scanner although its not intended as one. The keyboard controls and
buttons are rather intuitive and i like that fact that this radio has an all
metal housing compared to many of the other manufactures plastic.