QA Documentation for API650 Tanks

Why would you ask?

It is clearly stated in your OP. The minimum amount of documentation required is, "the QA documentation for equipment that has been installed on their site over the pass 3 years".

That would be any and all QA documentation for equipment that has been installed on their site over the pass 3 years.

It can't be simpler than that.

What the QA documentation for equipment that has been installed on their site over the pass 3 years was required by the original agreements for equipment that has been installed on their site over the past 3 years.

That's all they want.

Material specs, heat number etc. Welder qualifications, NDE results, PWHT records, that sort of things.

Such documents are prepared before mfg the item. Now how such documents can be prepared?. If it is back dated then it will be invalid.

Such documents should have been transferred during commissioning of the equipment.

Poor business practices here.