1958 Easy.
I have a 58 and 69, and the water guard appears to be the same, I have however noticed that a few years before 1958, they did change them. There used to be a bolt on the right of the suds-guard, directly to the right of the spinner opening. This suds-guard had a slightly different contour. The later 50's and up have L-shaped tabs welded to the outside of he spinner tub and the guard bolts directly down on those. All of this design seem to be the same.
On a side note, I was always surprised that the spinner guard was not porcelain. My 58 is in such good shape in comparison to the 69 that I though it was too until I took it off. The 69 is really a heap, it's a poorly built machine when compared to the older machines. My 69 has a worn drive-block and the overall build of the rest of the machine is nothing like the 58, which has weathered the time much better.
-Tim