Creating an image with untagged RGB 8bpc profile...how ?

There is no such thing as "plain old RGB". You CAN have, as you've seen UNTAGGED RGB files, but all that means is there is no reference as to how to interpret those RGB numbers. When you open that untagged file in Ps and leave it not color managed, Ps has to assume something, so it assumes the file to be in whatever RGB color space you have set in your color preferences. It's pretty much like using the Assign Profile command but without actually doing it and only having the choice of whatever you had in your color settings. The best you can do in this case, especially where you have erroneous advice from your industry forum, is to make sure that your monitor is calibrated properly, open that original untagged RGB file and use the Assign Profile dialog command to assign different color space profiles to the file until you've found the one that looks the best on your screen. Given that those untagged files were probably created in an application that was not color managed, they were, in effect in what's known as "Monitor RGB", and most monitors are either close to sRGB or Adobe RGB, your first two guesses are going to be the closest to being right. My vote goes to sRGB.

The other thing you might consider is how these files are going to ultimately displayed and on what devices. If these flight sims are viewed on a regular television then a color space like REC709 might be more appropriate. If they are primarily viewed on computer screens, then sRGB would be your best guess. FYI, REC709, which is the current commercial broadcast standard, is simply sRGB with a 2.4 gamma.