Allied Leisure was either the first or second company on the market with a solid state system in a game called Rock On. It was TTL based, and did not use a microprocessor.
Good photos for their game Dynomite, which is essentially the same game:
https://www.ipdb.org/machine.cgi?id=750
Mirco Games also came out the same year, but utilized microprocessors. Between the two of them, the release dates are unclear as to which one is actually the first:
https://www.ipdb.org/machine.cgi?id=2294
Just prior to that, Bally and Williams had also experimented with solid state systems, but didn't come out with games until after the two above-mentioned games were released.
The TTL-based boardset that Allied Leisure used was...complicated. What I think they were trying to do was replicate the workings of an EM game in digital form. An interesting note is that there are no diagnostic features because they were basically replicating EM workings.
A microprocessor-based system was a much better direction, and the way everyone else pretty much went at the time. Much simpler and much more compact.
Here are some of Bally's early solid state prototypes:
https://www.ipdb.org/machine.cgi?id=5103
https://www.ipdb.org/machine.cgi?id=4770
https://pinside.com/pinball/forum/topic/rare-prototype-atari
https://www.ipdb.org/machine.cgi?id=6338
And Williams:
https://www.ipdb.org/machine.cgi?id=5647
https://www.ipdb.org/machine.cgi?id=4828
Stern basically copied / reverse engineered Bally's boardset, so they don't really have early solid state development prototypes.
Atari had an early solid state system:
https://www.ipdb.org/machine.cgi?id=102
A few other lesser-known foreign early SS systems:
https://www.ipdb.org/machine.cgi?id=1997
https://www.ipdb.org/machine.cgi?id=3714
https://www.ipdb.org/machine.cgi?id=4870
