Used Microsoft Multimedia keyboard will work with your old transmitter
I can’t find any definite answer about this but I took a chance and bought a used Microsoft Multimedia 1.1 WUR00445 keyboard in eBay and I’m relieved it works perfectly with my old transmitter and mouse.
So if anyone is wondering, a used Microsoft Multimedia Keyboard 1.1 WUR00445 will work with your old Digital Desktop transmitter with no issues. Unlike newer models, the frequencies are not locked in. Simply connect it with your transmitter and it will work.
It was a complete waste of money for me, because I found out after I bought it that it will not work with the radio receiver and the mouse of the wireless system. They are sold in a package and you cannot substitute parts. So I spent $20 for nothing.