Arcades are all about custom chips. Most games have unique hardware and most even have unique chips. But chips that got used multiple times can open doors in a chain reaction. Let the Konami thread begin!

Contra uses two custom K007121 for graphics. The chip @furrtek is digging up at the moment. But was it the only game using it?

No! A bunch of Konami titles used it, one of them is Haunted Castle. So you get Contra, you get Haunted Castle... well, not that easy because HC also uses K007232 for sound

So we need to make a K007232. Only for one game? No, because TMNT also uses it. But that's not the only one

Ajax also has the same chip, along with a strange 6809 CPU variant called the "KONAMI CPU". That CPU is actually present in Haunted Castle too... Is it anywhere else?

Aliens share that same Konami CPU plus the K007232. So getting several of these chips right open up many possibilities. But what else uses that K007121 we started with?

We've got Trick Trap, which has a K007121 and it also uses another custom chip, the K051733 as a tiny math coprocessor. Was that chip used else where?
BTW, the FPGA core for this game is now in beta phase here https://www.patreon.com/posts/42557761 

Chequered Flag uses that same K051733! Not only that, it also uses the Konami CPU. But it doesn't have that K007232. Let's go back for a final example

Nemesis uses the K007232 chip as PCM controller. Same as TMNT, though they are four years apart from each other.

Slowly adding support for these custom chips in the easier titles, gets us to support complex arcade boards in the long run.
That's one reason I sometimes make cores for lesser known systems. The other reason is they are a lot of fun too!