If "blank back" means non engraved baseplates, it seems normal for pickups of the 80's. The very first / oldest Duncan's had oval stickers like the one shown in my avatar but a paper sticker with ink was the only indicator of the model otherwise (and both weren't systematically present on all baseplates: some had only one sticker, at least for those that I had in hand).
I thought the blue sticker in your avatar was used for rewinds and repairs? (Which is how Duncan started business - I believe the blue sticker is from before they had any Duncan models.)

Well, the oval sticker in my avatar has the same "S" logo than in this ad from 1979, mentioning "replacement pickups":
http://1.bp.blogspot.com/_Y_bcbohoK2...uncan+1979.JPG
Furthermore and for instance, my oldest 59's came either with a mere square paper sticker mentioning "59" (how surprising) either with this paper sticker AND the oval one with blue text.
EDIT - I've corrected the paragraph above because my old memory failed! I remembered paper stickers as saying "SH-1" but the text was "59B" or "59N" in dark red ink.
And here is another ad from 1979 evoking the "59" model aforementioned (first name in the list of humbuckers):
Red ink, wow, that would be the same time as my "The JB Model" which is also red ink. No winder initial on those? I think those would be Seymour-wound?
Approximate value. People on my lp forum are not very accommodating and rude
@hamerfan: thx for sharing! That said, the "no backlogo" mention seems misleading to me since most of my "prehistoric" Duncan's had the oval sticker with blue text pictured in my avatar: albeit removable, it is a back logo and it was clearly sticked on Duncan models, not on rewound pickups coming from elsewhere...
I mean logo impressed in the back.
The funny thing is I never saw a oval sticker in flesh in Germany. I always thought they are sign for repair or rewind. I think they are very very rare in Germany. Even my only pickup with a full Modell name (before 1979) came from an American guy living in Germany.
I also own 3 pickups with blank baseplate and only the white tag with modell ( 59N 59B), but they don‘t carry the oval sticker either.
So i cannot make out when and why the oval sticker was used.
I bought my first Duncan in 1980 and met the man in 1981 on the Frankfurt Music Fair.

