Fred K. Beckhusen has written a document The HP-35 “Bluebird” tester with many interesting details of how the HP-35 chips were tested - secretly, in the sense that Mostek applied a need-to-know basis for the operators.
The HP-35 calculator was a huge sales success in 1972, the year I graduated from high school. There is a lot of information online today about this marvelous machine, but very little about how it was manufactured, or how it was tested. I wish I had know back then what I know today, because I worked on the HP-35 for a long time, without even knowing about it. So here it is, 40 years later, and I am writing about the HP-35 chip set and architecture, and the Bluebird tester we tested them with.
This photo from the 1973 Mostek data book:
The HP-35 famously shipped with a bug in the ROM, and HP did a product recall - see for example this page on the late Jacques Laporte’s site all about the HP-35.
Images above from the very interesting Remembering The HP-35A by Richard J. Nelson.
You can run both versions of the microcode in this in-browser emulator by Ashley Feniello, who demonstrates both versions in this video:
via hpmuseum forums