The 1700s circuit board was identical to that of the 1750, and a trace marked J1 indicated the size of the chips used.
On the 1750 and 1764, this trace was cut.

The 1700 and 1750 had a resistor at position R4 that, according to Commodore engineer Fred Bowen, compensated for subtle timing differences in the expansion port on the C64 and C128.
The 1764 lacked that resistor.
Bowen and other CBM engineers recommended against using a 1764 with a C128 unless the resistor was added, or a 1700/1750 with a C64 unless the resistor was removed.

It was possible to check for the presence of a 1750 by reading memory address $DF00's bit 4, which was 1 on a 1750, and 0 on a 1700 or 1764.
However, since this procedure would not distinguish between a 1700 and a 1764.
 To solve this, programmers wrote to, then read from, the REU's RAM to find out the amount of memory installed.
The 1700s circuit board was identical to that of the 1750, and a trace marked J1 indicated the size of the chips used. On the 1750 and 1764, this trace was cut.
The 1700 and 1750 had a resistor at position R4 that, according to Commodore engineer Fred Bowen, compensated for subtle timing differences in the expansion port on the C64 and C128. The 1764 lacked that resistor. Bowen and other CBM engineers recommended against using a 1764 with a C128 unless the resistor was added, or a 1700/1750 with a C64 unless the resistor was removed.
It was possible to check for the presence of a 1750 by reading memory address $DF00's bit 4, which was 1 on a 1750, and 0 on a 1700 or 1764. However, since this procedure would not distinguish between a 1700 and a 1764.  To solve this, programmers wrote to, then read from, the REU's RAM to find out the amount of memory installed.
Commodore REU