Apple Performa 6400/180 - Manual (Page 227)

Additional Procedures 6500—Software Installation Errors and L2 Cache Identification
Identifying L2 Cache DIMM
If the logic board is an OPC, follow these instructions to
positively identify the L2 cache DIMM as compatible or
incompatible:
1 View the back of the cache DIMM.
2 Notice that there is one larger chip and eight smaller
chips. The eight smaller chips are organized in tow
groups of four at each end of the DIMM.
3 Look at the markings on any one of the smaller chips.
They will look similar to this:
IDT 74FST
32384Q (This number is the identifying factor.)
F9637BFU
If this number, not counting the Q or any other letters
present, has five digits, it is compatible with OPC.