Ow! Your asking me a good question that on the surface seems straight forward enough. It does lead to opening a can of worms though.BGRIN1
People have different reasons for collecting things. No good way to say for certain that one reason is best.
Some people collect M7's with a preference to their connection to the Vietnam War, and are only really interested in the one's that are period correct for that. Others may be interested in bayonets that represent Government contract items, and that would be inclusive of the Vietnam time period examples and others that were made afterward. Others have different criteria than those.
There are Imperial M7's with 90 degree runouts. As far as I know at this time the earliest contract from the Government for Imperial has a delivery date of 1973 and the next 1976.
The Government made an official change to the M7 blade runout in 1974. This is generally considered the start time for the radius runout. However, there are good reasons to consider that the Imperial 1976 contract had M7's with the near 90 degree runout too. Also, there are examples of many types of bayonet blades having radius runouts before this time. To make matters worse, many Government contractors seem to have switched back to the near 90 degree runout by the late 80's and into this century. Seems like we never really get to have things as simple as we'd like.
IMHO, if I was looking to add a MILPAR M7 Government contract bayonet to my collection, I'd stick with an example that had the near 90 degree runout. It's generally considered that the MILPARS with a sloping runout were likely made with leftover parts from a third party.
OBTW, IMHO, I'd consider the first two blades on the left in the picture you provided above as different examples of "near 90 degree runout" blades.