The ROM chips are labeled as follows:

U1: M
1818-0702
SCM91076P
Q7945

U2: S
7932E
1818-0703

U3: M
1818-0704
Q7939

U4: S
7952E
Ct1243
1818-0705

The "M" chips are motorolla, the "S" chips are, I believe, Synertek.  The numbers are just the HP part numbers and date codes.  According the service manual, the parts are "SYP2332" and are masked rom.

Before we travel too far down the ROM rat-hole, I need to get some cold spray and confirm that it actually a ROM problem and not something else nearby that I am heating.