Pre Sea-Gull Dongfeng brand wristwatch (1966-1973/74)

I bought one of these because of something that you wrote on another forum. They are a really underrated watch from this era.

I want one just to be able to use that case back tool!!

I agree. The folks at Tianjin Watch Factory took their craft seriously.

Single Sea-Gull/Dongfeng "factory" case back tools are occasionally seen. I've been told by experts that a set like mine (all four--especially the smallest one) is quite rare outside of the factory itself.

That noted, newer tools of the same design but without the plastic coated handle much easier to find if you know where to look.

That noted: I don't know why more companies don't go for that kind of back and tool--it almost eliminates the scratches that happen with the other types of tools.

That was also my first reaction. 😂

That noted: I don't know why more companies don't go for that kind of back and tool--it almost eliminates the scratches that happen with the other types of tool

I would think that it is too easy to cross-thread and over-torque with a hand tool like this. (I have a Chinese case of a different design, which came from the manufacturer in that state.) That's why I use a rubber ball on most cases. Expensive case tools have torque-limiters, as far as I know.

Fantastic 

I have two good rubber ball openers, and I use them as well, and they are also very safe to use. But they sometimes don't work on tight cases that open with almost zero effort with the factory wrenches.

These allow one to be very gentle when the fit is accurate, with excellent effectiveness. 

Cross threading when replacing the back (using either the polygon style or those with protrusions that match notches--see the photos) is easily avoided by just going in the opposite (left) direction slightly, at which point the caseback almost seems to drop into place.

I use this type of wrench quite often, on a good number of watches. I actually have probably 20 of them, of which about 10 are for vintage Chinese cases, here's my Sea-Gull/Tianjin Watch Factory set... (these were made for use by Tianjin factory workers)

...and here's a set of four for Shanghai and Zhongshan watches.

The only concern (and it's easily addressed) is keeping the case and back level, but that's an issue with most caseback wrenches, and with a rubber ball

I can't count the number of badly scratched backs I've seen that were plainly opened by folks attempting with these types (neither of which work that well on polygon backs) and I have decent variants of both.

Two point

Three point
 

Let's assume that I would open a watch with a rubber ball first, and also close it, if that is an option. 

Now, if the case is stuck, we are talking about a tool, and for a polygonal case back that is the polygonal wrench, just like Rolex-type case backs requires a particular socket.

The reason why I'm always hesitant about wrenches, is that the average persons tends to over-torque, and that is not limited to watches. Sure, you can avoid cross-threading by turning the case back the other way. It's almost trivial. Yet I come across items that are cross-threaded and over-torqued. (It must be human nature that If in doubt, you just try harder.)

Why other manufacturers haven't adopted polygonal case backs? I think the answer is simply that you need either a) a set of tools instead of one adjustable tool, or b) different manufacturer are unable or unwilling to standardize.

That all makes sense to me.