Sorry about the inconvenience.
OP's original ROM set 503/604 is invalid, has this been up already, can't really remember.
(still there for a moment and for the reference)
I have 3 sets on the table right now, 502/603 and 503/604 are original chips and 506/607 is self made, all are working copies.
My MiniPRO something can't read said 503/604 U24 chip, U23 is unknown.
I also just successfully copied 502/603 set with that same MiniPRO, destination being M28C64A, using AM28C64A MiniPRO type, and successfully used said 503/604 originals, and newly copied 502/603 EEPROM set, on the same controller.
So some weak bits then, but unfortunately only for the MiniPRO.
Disassembled software below is from 506/607 U24.
All versions seem to be starting equally, first jump address location differs.
That jump address also always includes, sooner or later, that delay of 0x452C cycles.
The faulty 503/604 U24 copy has at least one error, 18 in place of 1B DCX DE.
Somewhere is also a sort of a CRC checksum, but can't remember where.
It's in the middle of the code and it is included to the count.
0x0000 26 C0 MVI H, 0xC0
0x0002 36 FF MVI (HL), 0xFF
0x0004 C3 DB 02 JMP 0x02DB
0x02DB 3E FF MVI A, 0xFF
0x02DD 26 E0 MVI H, 0xE0
0x02DF 77 MOV (HL), A
0x02E0 24 INR H
0x02E1 C2 DF 02 JNZ 0x02DF
0x02E4 11 2C 45 LXI DE, 0x452C
0x02E7 7A MOV A, D
0x02E8 B3 ORA E
0x02E9 1B DCX DE
0x02EA C2 E7 02 JNZ 0x02E7
0x02ED C3 38 00 JMP 0x0038
E,
Changed a nuance.
E2,
Checksum
0x009C 11 00 40 LXI DE, 0x4000
0x009F 26 42 MVI H, 0x42
0x00A1 AF XRA A
0x00A2 12 STAX (DE)
0x00A3 1C INR E
0x00A4 C2 A2 00 JNZ 0x00A2
0x00A7 14 INR D
0x00A8 7A MOV A, D
0x00A9 BC CMP H
0x00AA C2 A1 00 JNZ 0x00A1
0x00AD 0E 95 MVI C, 0x95 ; Checksum zeroing value?
0x00AF 79 MOV A, C
0x00B0 B7 ORA A
0x00B1 C4 13 22 CNZ 0x2213 ; clear (DE) count C
...
; Checksum
0x0180 21 00 00 LXI HL, 0x0000
0x0183 AF XRA A
0x0184 86 ADD (HL)
0x0185 2C INR L
0x0186 C2 84 01 JNZ 0x0184
0x0189 47 MOV B, A
0x018A 24 INR H
0x018B 3E 40 MVI A, 0x40
0x018D BC CMP H
0x018E 78 MOV A, B
0x018F C2 84 01 JNZ 0x0184
0x0192 F5 PUSH AF ; 0x0C8C 3A 00 41 LDA (0x4100)
0x0193 F3 DI ; 0x0C8F E6 70 ANI 0x70
0x0194 CD 8C 0C CALL 0x0C8C ; 0x0C91 C9 RET
0x0197 C4 5F 15 CNZ 0x155F ; ext. I/O reset
0x019A F1 POP AF ; 0x02F0 F5 PUSH AF
0x019B B7 ORA A ; test result 0x02F1 3E F0 MVI A, 0xF0
0x019C 0E 08 MVI C, 0x08 ; 0x02F3 CD 85 05 CALL 0x0585
0x019E FB EI ; 0x02F6 F1 POP AF
0x019F C4 B8 02 CNZ 0x02B8 ; Error 0x02F7 3D DCR A
0x01A2 3E 02 MVI A, 0x02 ; 0x02F8 C2 F0 02 JNZ 0x02F0
0x01A4 CD F0 02 CALL 0x02F0 ; wait 0x02FB C9 RET
0x01A7 CD 83 06 CALL 0x0683 ; show module test results