I used the term "generic" description because I did not know initially the specific transmission. But the servos all function pretty much the same. My guess is that all the seals are showing their age and this one is just showing because it is external.
However, as I stated there are internal seals that go with the servo pistons as well. If your going in that deep you might as well replace those too. I say that with the caution that the internals of the servo can sometimes get complex - but it doesn't look that way for the C3 (see image). Also, depending on the available transmission tunnel clearance you might not be able to get the servo all the way out before it hits the tunnel. You should be able to find the seal separately, but sometimes they are only available in transmission rebuild kits (as in you have to buy the whole thing). As best I can tell you have a C-3 transmission. The part in the image for the external seal is 362. The servo piston (seal looks integral) is 902. Note these are NOT the part number, but just the number in the image for illustration. You will need to find the part by application and the box brand stores might not carry it. It is possible that you will have to go to a transmission parts vendor.