Hey guys,
Im looking for a frame to swap in for my whale of a Ross gran touring. Its a single speed and I have some decent parts on it that I want to put on a better frame. I'm hoping to find a decent FRAME for under $100 (more like around $50). I have two frames on my list that sort of fit my budget:

Centurion Ironman
Centurion Lemans

I know these bikes have sub- models or different versions like the Ironman Expert vs. the Ironman Master. For the Lemans there is the Lemans RS vs the Lemans 12. Do these "sub-models" share the same frame? I assume that there are some differences in components, but if I already have parts, I could just opt to get the cheaper one (if they share the same frame) right?
Thanks in advance.

If anyone wants to make any suggestions to add to the list please do so! I'm just looking for a decent lighter weight frame that handles and responds. if It has 27 inch wheels that is a BIG plus! Sorry if this post is a little much
Thanks a lot!

There is no hard and fast rule about sharing the same frame. Here is a discussion quoting BF's T-Mar on Centurion. The Ironman has 700c rims and early Lemans are 27".

https://sheldonbrown.com/centurion/

While brakes, cranks, derailleurs (oh forgot you are a SS guy), brake levers, saddles, and handlebars are usually transferable, you can run into difficulty on hub width, seat post diameter, stem diameter, headsets, bottom brackets, and shifters.

You might look for an early 1980's Trek or Schwinn Japan frame. Both are 27" wheels for the most part.

In the past lots of brands used to share one frame over a few models, Bianchi and Cannondale most easily come to my mind. I believe Schwinn did it a lot too and though I am unfamiliar with them I think trek did as well.

I think in your case you need to concentrate on just making sure your looking for a full chromoly steel frame and fork and it is built for 27" wheels.

Lots of brands used the same frame for differently dressed bikes.
Still others used the same geometry but different tubing.

I kind of question the strategy of looking for a frame only at this possible price / value /performance point.

Or, is this the leading edge of "fixie's" being turned back into multispeed road bikes?

Happened with many other brands. Just like the Gitane TDF and Criterium shared a frame in the 80's, both using the same Vitus 971 frame but the bikes came in different paintjobs with the Criterium in blue and the TDF in red. Peugeot did similar with their mid 80's PSV/PSN/Triathlon/Victory models with the same basic Vitus 980 frame, the only difference being colors, graphics component groups and an additional base-on fitting here and there for the Triathlon model...