It was a complete nightmare to remove the pins from my new ATX supply to rewire the plug, even with the right pin removal tools from ebay it took at least an hour just to learn a working method to release the pins.
If i had to do it again I would much rather buy an extension cable like in the instructions I linked, cut the wires that have to be changed and solder them back together while leaving the plug pins alone.
As long as you're not in a hurry the proper cable is quite cheap to get, something like https://www.amazon.com/24-Pin-Detachable-Extension-Conversion-Cable/dp/B01N5MWYX1/ with a 20/24 convertible plug and color coded cables makes the process easier.
You'll need to buy a new 120mm system fan as well, the original fan is powered by a custom pin header on the standard power supply. If you get a modern quiet fan your G3 will be almost silent. I am really happy with how mine turned out
A small bump, but I can report that, if you turn off saving passwords to the system keychain and the auto-updater, GyazMail 1.5.21 works fine in Mac OS X 10.3 through 10.14.
It's a shareware client that costs $18. The developer has had the wisdom to add sensible defaults, like turning off downloading images in HTML and triggering privacy-violating read receipts that caused a fuss a few months ago. Everything's customizable to a good degree.
I think it's worth it. Michael J. Tsai gave it a nice shout out in the midst of reporting on Catalina woes. I can confirm that it works well on modern IMAP with SSL.
That's probably the correct date. Check the dates on some of the other caps.
If your system isn't original then there's no telling when that board was installed.
Certainly Apple provided replacement parts for Plus's for several years after they officially stopped production.
They may have still been selling Plus upgrade kits in '92, I don't know.
(Apple also supplied re-worked boards as warranty/repair replacements.)
If you pull the board completely out and check the soldering on the back you can usually tell if the board has been re-worked.
There may also be a date penciled on the motherboard there next to the Apple Computer name. It would be the same format. (that white square to the left in your photo.)
Some of us settled on Mac OS X Leopard Server and VMware Fusion, in the absence of a solution like FuseHFS for the latest Fuse for macOS.
As far as mounting the disk images with a double click in Finder, for any given Mac OS X going at least as far back as when Tiger introduced the file system metadata system that Spotlight is based on, if you change the file extension from "dsk" to "img" then Finder will attempt to mount the image through DiskImageMounter.app. The underlying hdiutil ends up sniffing the file headers and some other bits on the disk image to figure out what the image actually is, it doesn't really care about the file extension.
The best solution for backwards compatibility on Mac OS is to keep some (virtual, actual) machines going with an earlier Mac OS X. As many are finding right now with Catalina.