http://www.cryptomuseum.com/covert/bugs/selectric/index.htm
Quotes:
"The data from the 6 magnetometers (i.e. 6 bits) was somehow digitally compressed into 4-bit words and then stored in a magnetic-core buffer that could hold up to 8 such 4-bit data words." "It is unknown why and how the data was compressed from 6 to 4 bits, and the NSA report is very vague on this point. It is possible that the Soviets used 4-bit logic and had to spread the 6-bit data over more than one 4-bit data word, but it is more likely that they used frequency analysis"
"According to the NSA report, the Russians compressed the 6-bit data into a 4-bit frequency select word. Although the report doesn't explain what they mean by this, we can make a few educated guesses. The reason for compressing it into 4-bits, was probably the fact that the Russians only had access to 4-bit digital technology at the time. The problem with 4 bits however, is that each data word has just 16 possible combinations"
...
Maybe a similar "hack"?