Does anyone have any info on it? As far as I know, no emulator supports it because most people don't know it exists. The dumps for Mappy, Lupin Sansei and others are all hax0red to MMC3.
I remember playing an early game by Namco with an obscure mapper, I don't remember the exact number, it was in the range 60-100.
I also don't remember the name of the game, but it was a lot like Zelda 2.
Could it be that mapper ?
Its name is Dragon Buster.
That would be the Namco 118 mapper, which is effectively a subset of the MMC3 (appears to lack IRQs and SRAM support, perhaps complex PRG/CHR 'flipping' as well).
That sound an awful lot like Tengen's Mimic-1 mapper, which according to Kevtris fits Quietust's description exactly. The Mimic-1 games all work fine on mapper 4, so the Namco games should as well.
The aforementioned Namco 118 chip is 28 pins.
The DxROM boardset (DEROM, DEIROM, DRROM) seem to use a 28-pin "109" chip, which may well have also been made by Namco, given that it is also an apparent subset of the MMC3 and is very similar in appearance.
Many people assumed Tengen's MIMIC-1 mapper (also 28 pins) was a clone of the MMC3, though it looks much more like it may have been a clone of the Namco 109 chip. If anything was a clone of the MMC3, it would have been the RAMBO-1, though that also had some significant modifications.
Hmm maybe the MMC3 is effectively an enhanced clone of it! Thanks for the pointers!