Ezycoat informs me that, based on his experience and knowledge, the last year SEIKO used "Waterproof" was in 1970, with some exceptions in 1971. The reasons for this change should be obvious, as anyone who purchased a watch marked "Waterproof" that later experienced water damage, would no doubt be somewhat upset. Here are some examples that appear to confirm this information...
A rare 27J 4005-7000 from November 1968...
Note "Water Proof" on the caseback.
Considering that the 4005 movement was used only with the earliest Bell-Matics (and features the date-only dial), if you ever found a date-only Bell-Matic with a caseback other than 4005 (and "Waterproof") then you could confidently say the caseback was not original.
Here is a 4006-6010 from 1969...
Observe how on the 1969 4005-7000 it is "Water Proof" but on the 1969 4006-6010 it is "Waterproof."
Does the 4006-6030 17J "cushion case" from 1970 pictured below conform to this information?
Let's take a look at a 4006-6002 17J Bell-Matic with a 1972 caseback...
Note the placement of the "water resistant" imprint on the caseback of this NOS 4006-6031 from 1976...
Now, to keep things interesting, here is a puzzler. Could the 4006-6070 7O0081 Bell-Matic pictured below be evidence in favor of the 1977 date for the end of 4006 production?
My first assumption would be a production date of October 1967 (1967 being the generally accepted first year of Bell-Matic production), but "Water Resistant" on the caseback tells us something altogether different. The only logical assumption to make is that this -6070 style came late in the Bell-Matic manufacturing run and establishes that Bell-Matics were made at least through 1977. However, the 4006-6061 shown below indicates a manufacture date of 1978, further adding to the mystery.
I hope you find this posting of some use to you in your effort to learn more about your prized Bell-Matics.
