The original clamp plates for the AD trucks, and I'm sure other years, do not have countersinks and do use flat bottomed nuts. The clamp plates do however have some shape, raised portions under the nuts such that the clamping pressure provided by the nut is spread over a larger area/diameter.
Some of the plates in your picture look like may be torched out centers of old wheels used as spacers for whatever reason.
Clamp plates used with this same 10 lug on 7.25" circle (but 5.25" bore) from the 80's and later (possibly earlier) do have countersinks and do use 90* lug nuts.
Same details as above are true for the 1 ton wheels and the corresponding 8 lug clamp plates.
If necessary I could take pictures of both types.
As to using them or not.. if you read the Accuride wheel catalog the modern equivalent of these wheels require use of clamp plates and they provide the GM part number for them. Similarly, if GM used them on our old trucks when new it's safe to assume that they did so for a reason and it'd be smart to continue using clamp plates.
