How to date an M7 bayonet?
 

Hi,

I am looking to buy the best condition M7 I can find, from the VIETNAM era. There are a lot of them on eBay from different manufacturers, and I don't know how to identify their ages.
Is there a way to know this? Like... certain companies only manufactured them between certain years?
And how about ones that come with the M10 scabbard/sheath, were those used during the Vietnam War? Which year was that introduced?

Thanks in advance for any help you can give me...

Scott