Nobody has ever found the 2006 or 2005 clients, they are just modded hacks. That exe file "RBLXDynablox.exe" had some misspelling on that, as "Dynablox" was not even close to what it was named before, "Dynablocks." I will do some research on Wayback Machine to find some actual things, maybe a download for the version, i will do the research, and update for you guys to check it out. Plus the exe name file had "RBLX" short for ROBLOX, so that exe file might be a recreation or a mod from some user.
Update: Stumbled upon a blog for the request, there had two links, both of them deactivated.
Stumbled in lost media wiki, that offered the original setup.exe from the broken website on the wayback machine, follows the link below
web.archive.org/web/20120712141451/http://flaky.eu/roblox/RobloxSetup.exe
The setup will automatically download. :)
Status is still blank, updating in a few minutes :)
YREEEEEEESSSSSSSS, The download worked.
Only the client works, the Play ROBLOX app is totally unusable.