I am always amazed how islands of projects can co-exist with little crossover. I just found this thread after several years of working on 884X projects through the Groups.io forum for Fluke hardware. I have more than 20 of these meters and some with Z86 issues, 700013 issues and bad displays. Several years ago I too found the UB8840 clone John.ccac built for a disk drive. That had me motivated to design such a clone for the 884X.
The posts in this thread are not absolutely clear on the ROM versions used. But it should be clear that 8840A, 8840AF & 8842A all use different ROM versions that are not cross compatible. My UB8840 clone worked with all 8842A ROMs, but not all 8840A ROMs. In fact, only the 8840A V4.0 ROMs worked flawlessly in the 8840A meters. I have a suspicion, but not confirmation, that there is a timing difference between the Z86 and UB8840 such that data transfer to the display controller is incorrect. There is likely timing differences in older ROMs that are improved in the V4.0 version.
As mentioned elsewhere, if you can find a Z86E21AF microcontroller with a glass window, it can be programmed with the complete 8K ROM, though I have only verified this with the 8840A V4.0 ROMs.
Because the 700013 quad analog switch is the most likely failure point in these Fluke meters, I have, over the last 2 years, build 5 different clones in an attempt to duplicate this unobtainium part. I began with a Microchip PIC and DG212B. This processor has 4 programmable logic blocks independent of the program counter. It worked, sometimes, in some locations. Unfortunately, some obscure timing or noise issue kept it from working universally. But it would have been a easy 2-chip solution.
I made 2 other versions using 74HC & 4000 series with single chip inverters and a DG212B. These also worked, sometimes. And were a bear to assemble.
The 5th version used a PLD and a DG212B. It is also a 2-chip version, much easier to assemble. This version was condensed onto a PCB about 30% bigger than the original IC. It works flawlessly in U301, U302 & U303. Should also work in U40X positions.
I have made this available as a kit or fully assembled in limited quantity. (I need to recover some of the expenses incurred)
https://www.hollywoodcontrols.com/phpFluke/HC700013P.php