Having seen backs mis advertised on ebay I thought a little knowledge might be a good thing so have endevoured to clarify to some extent the history, dating and what to look for regarding faults and evidence of use.
I have for the purposes of this only included roll film backs leaving out Polaroid, Digital, and bulk film such as the 70 on back as they are a whole different chapter.
The magzine or for the porposes of this guide will now be refered to as back was first introduced in its current fittin in 1949 to fit the 1000F and the 1600F and has had during its life several modifications the most major being in 1968 which I will describe later.
First came the 12 on in 1949 which ran to 1954 and then the 12 on which ran from 1955 to 1968 , both of which were simular in design and were loaded by looking through a hole in the back - winding unti "1" became visible and the reversing the counter to locate frame 1 for the purposes of use.
How can I tell them apart well that is simple - on the bottom are two small windows the one on the left being the frame number and the one on the right being the exposed indicator (red exposed - white not so) . On the 12 on the film number window is to the left of centre of the wind on , wheras on the C12 in is withim 1/4 inch of the exposed window to the right of the centre of the wind on , which incidentally is still where it is located on the latest design.
This back only came in 12 and 16 on deriatives but at one time could be used with 220 by resetting the counter after shooting fram 12 and using a rubber plug in the hole at the back, however, don't fall for the bit that tells you that it is possible now as its not as the plug has not been made for years and if you find one its going to be either hard or perished and 220 film has no backing paper so it will fog unless you tape up the rear cover with duck tape.
The A12 was introduced in 1968 the A meaning Automatic setting to frame 1 and untill today utilises thje same loading method - yes there was the E type introduced with electrobus connectivity in 1994 but unless you are using the F series 201F etc.. you would not need them but remember the E type will work on a 500 series but non E type will not electronicaly couple to the F cameras.
Dating V H P I C T U R E S read 1 2 3 4 5 6 7 8 9 0 ie RS is 1980
The most common fault is the light seals - not an expensive repair but in 99% of cases caused by damaged ldark slides (sheaths) so if you see a magazine advertised with a damaged sheath its odds on the light seal has gone or if no dark slide the chances are it was thown away for that reason and the same would most probably apply.
Next problem is that the film wind on gets a little on the stiff side this is in a lot of cases down to a small nylon ring which acts as a bearing on the wind on and is inexpensive to repair but can also be down to a need of a service / lubrication.
Third problem is that they have a tendency to fail to stop at frame 1 when winding in the film or mis space where in some cases you end up with irregular gaps and only shoot 9 frames although 12 register and the last 3 are fired on the backing paper - not so great if its the cake shot at a Wedding. My suggestion is if you have any old out of date films or see tham for sale load one in and mark fram 1 and then advance , shoot, remove back and slide and mark each frame in turn to confirm spacing, yes it may be a pain in the rear but can save problems later.
Evidence of Use
First place to look is on the mounting surfane where the serial is stamped as it wears first and is a good guide zs to how many times it has been on and off of how it has been stored ion a kit bag and remembering that while you are checking the serial number the last three numbers are also repeated on the insert - When new thay are paired at the factory and wear together so if mismatched at a later date can cause problems with spacing , my advice would be to avoid mismatched backs and if the numbers are not on the insert ask why ?. I have over the years seen chrome backs with black inserts and vice versa which I personally would not entertain using.
On the rear of the early backs 12 on anc C12 the film reminder and rear cover are one and should spring shut to the rear of the back if its sloppy or missing draw your own conclusion , likewise check for damage to the plastic film reminder dial on the A12 and remember if its missing its not too inconveient or expensive to repair but there should be a spring steel plate for it to mount to, if this is not there it was attatched by two small rivets which if not there is a light leak.
On the A12 the wind on knob was prone to loosing its outer part with medium to heavy use (the small part you hold whilst winding on) and in some cases may even be missing - consider if this is the case - why ?.
From around the mid 90's the release button on the back lost its -V- marking which stood for Victor and gained a number representing the exposures on the film , and the latest gained a handy dark slide holder but as a word of caution if this is in any way damaged it is secured to the magazine by 4 small screws directly through and if they are loose you have a potential light leak.
Derivatives A12 (12 on 6x6) A16 (16on 6x4.5) A16S (12 on 4.5x4.5) A24 (24 on 6x6) and A32 (32 on 6x4.5) , beware the 16S which was superslide to allow you to project transparencies in a normal 2x2 projector and remember the mask so you dont fill the frame.
A final note regarding Russian backs - yes thay do fit but personally I would not use tham as they are all the early pinhole 12 on or C12 type and iy your film plane is as little as 0.25mm out so is your focus.
Hope this helps - good luck with your purchase.