ShortScale.org

Looks to me as though there's one of those black plastic plates under the neck plate. Why/when were they introduced?

Modern DiMarzio's have a Letter and a 3 digit code stamped on the back. All I can see from your pic is a "F" at the top and a "8" down the bottom. All I know is it doesn't look like the old failthful Super Distortion that got retrofitted to nearly everything.

If you send a description of it to DiMarzio's tech support they will usually be able to id it for you.
Have a look on their FAQ What is this pickup?

My 1973 Bronco has the plastic thing between plate and body. Pure speculation, but I would assume they started putting them in when/after they changed from using nitro to poly. I don't think nitro likes having certain types of plastics/rubber. Maybe they were fitted to prevent finish cracks from (over)tightening of the neck screws?