There is some interesting reading about the Thunder Tiger - OS Max - Magnum trio.  If it's all strictly truthful or not, is another thing.  The version I saw originally included a named individual, though, which made it seem more like a real story.

It went like this: the chief engineer at O.S. when the FP and FSR were designed was a Taiwanese citizen, and he returned to Taiwan to lend his talents to the company there that would be bringing out the Thunder Tiger.  He or someone decided to clone the OS originals as closely as possible, so parts swaps were possible.  The TT engines had the same running characteristics as the OS engines. 

When some import contract was up for renewal, the mainland China company for the Magnum enginies recruited the same engineer once again, but this time the clones were not quite as exactly interchaneable with the originals.  The running characteristics, however, remained very similar to those of the OS Max engines.

It is a very plausible story. 

One-eye, your story may be correct regarding the mainland-made Magnums.

However, there was a time when Thunder Tiger and Magnum engines came out of the same factory in Taiwan. The last of the iron piston engines and the first series of ABC/ABN? engines with the GP prefix for both brands are the engines affected. I have examples of both to confirm this info.

The mainland-made Magnums appear to be different to the latest Thunder Tigers but I cannot be more positive because I do not have any of the latest models.

Cheers

Geoff