The three approval bases
Every PMA is approved against one of three bases. The basis tells you what evidence supports the approval, and therefore what you can expect to be able to ask for.
Identicality
The article is shown to be identical to the type certificate holder's design. The evidence is a comparison against that design, and the approval is supported by documentation establishing that comparison.
For a buyer, this is the basis that most closely resembles the same part from a different source.
Test and computation
The applicant demonstrates that the design meets the applicable requirements using test data and engineering analysis. The article need not be physically identical to the original.
For a buyer, this is the basis where a discussion about performance and qualification data is most likely to be relevant.
Licensing agreement
The approval rests on a licence or agreement with the type certificate holder, allowing production of the article. The approval record commonly references the agreement and its date.
How to use this in a purchase
Ask which basis applies to the specific part number you are buying. It is recorded in the approval data held by the issuing authority, so the answer is verifiable rather than a matter of the supplier's assurance.
Sources
References are provided for orientation. The applicable revision in force at the relevant date governs; confirm against the issuing authority's current publication before relying on any statement here.