Currently our base specifications do not reference versions of the spec, but only the parts. Same is true for the template and many released companion specifications.

In the beginning of the references is stated
"The following referenced documents are indispensable for the application of this specification. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies."

BTW: For example Part 3 and 4 have different text, I have not checked all parts. I think the text should be the same in all parts.

Those references work in a way that if I use the latest version of OPC UA with a companion spec it shall work with the companion spec as we are supposed not to do breaking changes.

What we loose with that kind of references is the knowlegde, which older version of OPC UA would be enough to support the companion specification. I do not think that we have Profiles on a level that it would help for this purpose. For example, for Ammendments 1-3 we have no additional profiles defined (should we do this in the future?).
The NodeSet-Files of the companion specs do reference versions of the NodeSet-File of the base specification. Is this enough information? And if yes, should we at least point out in the companion specifications that this is the intended way. Do we provide information to figure out from the base specification NodeSet-File which version of the spec including Ammendments are included? If not, should we provide this (e.g. as comment in the NodeSet-File)?