Perhaps the knife dug up at BirGorkha wasn't as old as guessed. Remember that some kamis give estimates of khukuris being 10,000 years old while actually being from the mid 1800s. The idea that the early khukuris have a very rounded shoulder or no shoulder at all is sound and still a good guideline.
The Civil War photo turned out to be a recent shot made to look old of some re-enactors and was published in the Dixie Gun Catalog(?). There was a lot of discussion on this forum quite a while back so the thread exists somewhere.
The booklet I gave Bill was something I picked up at the Gurkha Museum. It may be available from them, but last time I was there they hadn't made anymore up. It has a lot of information including the shot of the Gurkha troops in formation with Brown Bess muskets and attached khukuri bayonets. It says the date was before photgraphy was available (according to Beaumont Newhall; The History of Photography)so who knows.
I have seen 2 versions of what the khukuri bayonet is supposed to be and 2 documented drawings that it really did exist. These are from a 1976 article in Guns Review by J.A. Carter.
There are copies around that may be original, and copies that are fakes. The only ones I would trust are the 2 in the historical museum in Kathmandu and if I could find an example of the documented one. No one has yet found mention of these bayonets in store's journals, uniform regulations, or old standing orders for troops.