These chips are the DOS ROMs that occupy the various versions of the 1541. 
The early drives use two 8K chips, namely 325302-01 and 901229-xx. Later
drives were cost reduced (fewer, more integrated chips). The 1541C (short 
board 251854) use a single 16K ROM: 251968-01 or -02. Similarly, the 
1541-II uses a single 251968-03 DOS ROM. There is a version 4 1541-II 
that uses a 355640-01 16K DOS ROM.