No, it's definitely MPF553 on the package and the die. I think the only differences between the 1400 and 1670 are the voltages (which don't matter to MESS) and more outputs. The 1670 die looks identical to the 1400.

I'll look at the 1990 die and figure out the pins. Almost always they are in order, numbered counter-clockwise. So if you can find power and ground, you can usually get the rest pretty easily. I posted a cleaned up die shot for Little Professor 1978, and I took pics of the TI-1000 die.