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A24 or 24 film back How do I know what I am buying



Is there a cosmetic indication for the A24 and 24 film backs? How can I tell the difference.
I was looking to buy one.

I currently have 2 120 film backs.
I dunno if they are A12 or 12

Is there a reason why I should look for the A version?
The A-backs have a folding wind crank on the counter side. The non-A magazined have a key similar to the one on the opposite side, or an accessory non-folding crank like thing.

Plus the dial on the back of the back folds open revealing a peep-hole going all the way through the pressure plate, allowing you to see the numbers printed on the film's backing paper.
The A-backs have a mem-holder, also folding open, but revealing nothing but a plate spring holding the thing neatly shut.

A-backs are more convenient in use. And a lot newer.
But if in good shape, the non-A backs are excellent to use.
They are both the same as far as the important items.. like film flatness and spacing issues?
Yes, except for loading (a crank that stops at frame 1 instead of visual inspection of the frame numbers on the backing paper), they are both the same.
What style back is this?
What is the black button for on the side?

Is this a A24 or 24 back?
Ah! After some contrast adjustment i see what you mean.

I don't know. Though i remember (have to look it up) how at one time they were experimenting with a double exposure button on the magazine.
As far as i know, such a thing never made it beyond the concept, perhaps prototype stage, and was never available to the public.

But it could be something else completely.
Even something someone stuck on for a joke.
These photos are from my wonderfull $1600 DSLR
I forgot to adjust the flash expsosure +1.
You would think I would not have to do this....
Th button can be seen the best in the second photo
It is at the 9 o'clock position
It is between the chrome steps.
The store I bought it from insists it is a A24 back.
because the rear peep site in the back does not open. It looks like it would open, but it is screwed in place
That's the thing with non-A 24 backs.
None of them can have the peep-hole construction, since there is nothing to peep at. Just film to fog.

I will have led you astray, i think (must see what i said exactly in the what-are-these-for thread), by mentioning something about the 220-plugs and 24 backs. The plugs are to plug up the hole in the non-A 12 backs when 220 film is used in those.

And again, in this thread, i shouldn't have said what i said, at least not how i said it: there are no "non-A" 24 backs, except the Automatic ones that do have the type of wind key we see on this one, instead of the folding crank that were put on all A backs later.

The button is still strange...
I had a look, but it looks nothing like the double-exposure switch i mentioned earlier, though it could of course still be something like that, just looking different.
Then again, it could be something else completely.
In short: i don't know.
The mystery button on the 24 film magazine looks like the modification that John Kovacs - the venerable and respected Hasselblad repairman - has been doing on film backs for some years. By pressing the button down, the wind gear for the magazine is disengaged and a double exposure can be done. Easier than the usual way of removing the film back, winding the body and re-attaching the film magazine.
Just for fun, this is a part of Hasselblad's 1982 double exposure modification. The one we never got.


The figures on the left show a 'wheel' popping out where in Pete's photo the button is. Rotating the wheel (lower figure) makes it catch the wind gear and pull it out of engagement.
The figure on the right shows a 'head on' view, making the profile of the magazine more visible, and in doing that reading the figure on the left a bit easier.