The Moden/100, Moden/36 and Alert Central paging encoders are pretty much the same thing with little difference in features. They all are covered by the same service manual. It is only capable of 6 tones in the PROM.
The tones encoded are are controlled by ROM's U22, and U28. These are Harris Semiconductor HM1-0186 Commercial Diode Matrix. These are a 6x8 diode matrix, which adds up to 48 diodes on the inside.
The links of the diodes inside represent the tones. There are tables in the manual that show how the bits should be.
It's in a 14 pin package. These had to be pre-ordered in bulk from Harris. I'm sure this was no problem for Motorola when they were selling these new. But, now that's not the case.