Why bits are mixed up (interleaved) during ATC transponder mode A and mode C reply format?
Mode 1 transponders only had six bits: A1, A2, A4, B1, B2 and B4, which clearly wasn’t sufficient to identify specific aircraft.  Each day, all pilots would be given a common code to squawk when returning so the air defense folks would know they were friendly, whereas a foe would only have a 1/26 chance of guessing correctly.
Mode 3/A upped this to twelve bits, adding C1, C2, C4, D1, D2 and D4. However, the C bits were cleverly interleaved with the A bits, and the D bits with the B bits, at twice the transmission rate. The result was that (for instance) Mode 1 code “12” and Mode 3/A code “1200” looked the same, at least to the primitive equipment of the era. This was specifically intended so that Mode 1 aircraft could work with Mode 3/A radar sites and vice versa, allowing a gradual upgrade. Where the new equipment was in use by everyone, individual (“discrete”) codes could be assigned to each aircraft.
This is also why, to this day, xx00 codes are mostly “non-discrete”, i.e. they have a general meaning rather than identifying a specific aircraft.