LuckyCipher,
Welcome to the Shopsmith Forum.
Although the sticker says Serial Number it is a Date Code and yours was made March 23, 2006. Apparently, the Bandsaw was a popular accessory for the Mark V 505 as I have one made April 28, 2006, that I bought used and also came with a Bandsaw.
Hopefully yours came with the Owner's Manuals for the Mark V and Bandsaw. If not, I recommending getting them. A PDF of the Bandsaw Manual can be downloaded from the link below from here on the Shopsmith website. That one is for the newer Bandsaw with the Aluminum Table. A Mark V combination 520, 510, 505 Owner's Manual can be purchased from Shopsmith and is part number 845525 for $29.99. You could, as an option look for a used manual. Getting the combined manual would have you covered if you decide to upgrade.
https://www.shopsmith.com/_files/ugd/d5 ... 205b8c.pdf
As for the Power Coupler (part number 503574), Lathe Drive (P/N 505715) and Drill Chuck (P/N 505633 with key) they are easy to find. All three can be purchased new from Shopsmith through this website. They can be found used but a word of caution when buying a used Power Coupler, do be sure of its condition. The Power Coupler dates back to the mid 1950's and older ones could break due to age/condition. Any of the Drill Chucks made for the Shopsmith Mark models over the years will work. They all have a 5/8" bore to attach directly to the Quill's 5/8" reverse tapered Spindle. The Jacobs 3326 Drill Chuck from the 1950's is a popular one. The current Drill Chuck Shopsmith supplies (manufacturer unknown) is very similar in looks to the Jacobs 3326.
An upgrade that is easy to add is turning your Mark V 505 into a Mark V 510 with the addition of the 2 Floating Tables, 4 Connector Tubes and 2 Adjustable Support Legs. That is the difference between the 505 and 510 models. Floating Tables to make it a 510 would have to be found used since the Mark V 510/505 Models, like the Mark V 500 have been discontinued. As an option you could upgrade to the 520 model with the Pro Fence. It is the same Tables but a different Fence and Fence Rails.
Depending on what is broken on the Lower Saw Guard you could buy a replacement part to repair it. From the Home page menu look under
Shop and in the drop-down menu
Parts.