6 digit and 7 digit serial numbers.

The early Bell-matic's 1966, 1967 and some examples at the beginning of 1968 have 7 digit serial numbers. The first two digits being Year and Month.

From late 1967 (early 1968) to the end of Bell-matic production Seiko only ever stamped a 6 digit serial number.

Due to the relatively short period between late 1966 and early 1968 this makes the 7 digit serial number rarer to find.

4006-7000 produced May 1967