Just want to point out that the 4x74273, 1x74174, 1x7404 circuit isn't exactly the same as the NEO-273, the TTL circuit can address 32M and IIRC the 273 can address 128M. Since the IC is just a bunch of DFFs to latch the address for the CROM and SROM it's pretty easy to derive the extra bits (and the last bits for 512M and 1G CROM). Basically on the rising edge, the CROM A bits are latched, on the falling SROM are, and that's all there is to the chip.
BTW, in case anyone's wondering the PROG board's TTL circuit is NOT equivalent to the NEO-PCM, it's completely different; the early boards have separate ROM bus for each of the YM2610's ADPCM-A and B ports while the NEO-PCM services both ports with one ROM bus.