Those parts rifles may be from a rare arsenal or series which would explain why they sell for so much.

Kokura arsenal made some rifles without a serial number on the front band but they were proof marked for that arsenal so they would t appear in a Nagoya or Toyo Kogyo rifle. There is much to know about the rifles other than simply slapping on a donor stock and metal parts. True collectors know the difference and are not interested in put together mummies rifles simply because they are not correct and never will be. Sure lots of folks spend way too much on jumped up rifles with remembered parts, mismatched wood and metal, etc. but once you know the difference, you won't want a rifle that has been screwed with.

It's always best to save your money for a real matching and mummies rifle. In fact many folks spend more trying to restore a rifle than they would have spent on a good one in the first place!