I just saw this video recently, and noticed there's not much discussion here about the Nonlinear Systems Touch Test 20 DMM. It was quite interesting (about 1/4 of the mailbag video was about it, from about 33 through 45 minutes).
Dave talks about its microcontroller brains at 40:04 through 40:35. There are three rows of characters marked on the DIP-40 package. Top line: 51-126. Middle line: B8647-11. Bottom line:8052, and some sort of logo, maybe Motorola. Dave's first thought in the video was that the middle line of text was the model number of the chip. My first thought was that it was the bottom line, an 8052 processor. But, on the schematic at 44:32 through 44:42 in the video, the chip is labeled as 51-126 (the top line of text on the actual chip).
I'm thinking the 8052 printed on it is probably a date code. Most of the components on the board lack date codes, but there are two other similar ones I saw - 8032 on the big blue resistor and something else similar on one of the logic chips after Dave pulls one of the boards out. A web search does not initially reveal any microcontroller known as 51-126. It might be part of the 8051 family of microcontrollers.
The logo on the chip is most visible at 40:09 to 40:11, but I still can't make it out, It's very faded, and I'm not familiar enough with it to know for sure what it is.
At 43:25, "You'd get so excited if you had this back in 1981, let me tell you! As long as you didn't open it."
![Laughing :-DD](https://www.eevblog.com/forum/Smileys/default/smiley_laughing.gif)
Yeah, the PCB layout is pretty wacky. It's like the company had one designer who had everyone else convinced he was the only one who could do it, but he had a lot of logical faults behind his decisions.