Piggyback microcontrollers.

So, what is a piggyback microcontroller?
A "piggyback microcontroller" typically refers to the practice of adding an additional microcontroller to an existing electronic circuit or system. This secondary microcontroller is often used to perform specific tasks or functions that the primary microcontroller cannot handle efficiently or for tasks that require isolation or specialized processing.
Additional microcontroller integrated into a system to augment or enhance its capabilities, address specific requirements, or provide isolation and redundancy. This approach is common in embedded systems and IoT (Internet of Things) devices where different microcontrollers can handle various aspects of a complex task or system.

During the 1980s, a significant number of high-volume microcontrollers were either one-time-programmable (OTP) or came with factory-programmed mask ROM. Developing code for these microcontrollers presented challenges.
As a result, the 'piggyback' package emerged - in this approach, the CPU was placed in a package with a socket on top for an EPROM. This configuration simplified the development of code for the processor, and EPROMs could be easily swapped and erased as needed.

Example, National Semiconductors NS87P50UD + EPROM AMD AM2732 on ADT security board:

Currently old “vintage” piggyback microcontrollers are collectible items (affiliate link here but you can find it in many other places on the Internet) - https://ebay.us/UU5oZZ

So, if you have any, there is a good chance you can make some money !