Its fairly common for a manufacturer to make several devices out of a single actual die. Just disable part of the die, whether because its faulty, or not needed, or simply do not connect the pins to that feature. Intel did this a lot with the Celeron, and PIII line, disable some L2 cache on a PIII and you get a Celeron. Today it is done with multi-core processors.
Using a common wafer for several products saves a large amount of money, no need for a second mask set, and testing systems. Here we have a Texas Instruments TMS320E17JDL. The TMS320 is the industry standard in DSPs (Digital Signal Processors). The E17 from 1990 runs at 20.5MHz has a 4K EPROM, 256 bytes of RAM, and a pair of serial ports. You can see the large sections of the die devoted to the ROM, RAM, and MAC (Multiply and Accumulate).
This is the TI TMS320E15JDL. It is the same basic DSP core as the E17, it includes the same 4K EPROM, the same 256 bytes of RAM and the same MAC unit. It has some I/O ports tasked with doing different things, but thats a relatively minor difference. The big difference is the E15 lacks the 2 serial ports of the E17. You can see on the die where that hardware does not exist, its a large black spot, void of any circuitry. A very interesting and unusual occurrence.
TI either used a completely different mask for the E15, or they simply chose to not expose that small part of the mask.