I had never really looked at the shapes of the spent shell chutes, but I'm wondering if the difference isn't more a matter of who made them, than when they were installed. Because frankly there does NOT seem to be a time difference. The early built USS BLACK has what Steve (Model Monkey) has labeled as "late model" chute (with the circular clean-out plate?) as completed through 1970's, but the late built USS CUSHING (DD-797) and USS LITTLE (DD-803) have the type of chute that Steve is calling "early model".
For those not aware of it, when the first FLETCHERS were built, there wasn't a chute on the aft end of the mount. There was a door. That was replaced by the chute during Fitting-Out at the Navy Yard they were assigned to. Below are images of USS O'BANNON (DD-450) as completed in June 1942 and a view of USS NICHOLAS (DD-449) in January 1944. Both were fitted out at BosNY, and the modification to the shield would have been done there. As you can see, another variant for the chute. Also, note the ladder locations on these two mounts!!
At every yard period, there were always a long list of OrdAlts for changes or upgrades to the various weapons and fire control onboard the ship.
Meanwhile, at NYNY when USS FLETCHER was Fitting-Out in July 1942, she had the chute installed and with a "squarish" clean-out plate. Then in the second close-up view, you can see that where the braces or whatever they are for (I wonder it they are water seals, because they had a big problem with seawater entering this area where a lot of the electrical and hydraulic equipment was located) would later be located line-up with seams.
Then in August 1943, at MINY, FLETCHER's chute is the same and we get a view of her gun mount face with "braces".
This doesn't answer the question about USS BLACK's gun mount configuration, but does show that Steve (Model Monkey) and Benjamin (Black Cat) could be making a multitude of variants which are only a bit different and are hard to see in 1/350 scale. There are always a multitude of minor differences between various units in each class of destroyers. When I personally model a destroyer in 1/700 or even 1/350 scale, I don't worry about little things that would be hard to see or even notice. I concentrate on he bigger things, like bulwark variations, sensors installed, the Gun Captain's Hood, etc. Both the Tamiya and Trumpeter FLETCHER kits have plenty of errors that need correcting, replaced, modified, or simply ADDED because it is missing to be accurate (both Tamiya and Trumpeter don't have flag bags or a boat winch!!!). In the larger scales, like 1/144 scale and 1/96 scale, then what can be seen and noticed grows. Each modeler has to decide what is GOOD ENOUGH.
It appears from the images I have of USS BLACK and that show the mounts, that the three images below show what her basic configuration of her mounts at the end of WWII into early 1950's were. Call them what you want, but there really was no "early and late", except for the addition of braces on the front face, handrails along the side of the mounts, and a hood for the Gun Captain.
By the way, this image of USS CHARLES K. BRONSON (DD-668), shows that some of USS BLACK's sisters had hefty vertical reinforcements added to the 51 mount late WWII, because of damage from heavy seas. Something that was more common on the SUMNER and GEARING classes. Also, the "metal Charleston Navy Yard" style sliding metal covers for the gun port was being added on many units at the end of WWII. But, not on USS BLACK.
Also, note the 5-in/38 gun being worked on alongside BRONSON, provides details without the shield installed. You can see the equipment that the access plates on the right side are there for.