As the OEM they would have made a quick and dirty assessment the part would do the job and go from there.
Is true. However, even though for certain parts the requirement to test might have been minimal to nil, the parts still had to be documented within the design of the aircraft, and put through the approval process with the FAA. Thereafter, the OEM had to regularly audit the manufacturer of those parts, to assure that they continued to conform to what had been approved (auto parts manufacturers tend to change things without telling their customers). With the parts at OEM receiving, they had to be inspected, "batched" into their airworthy parts stores, and maintained there for however long they took to sell.
If the OEM bought a hundred door handles in the 1960's, bore the cost of all of that process for them, and then sat on half of them as intended spares for all those decades, continuing to carry the cost of annual inventory, and then budgeted for the cost of warrant or liability on those parts after they were sold and documented with a Form 1, there can be quite a cost to that!
Or, the OEM could choose to no longer stock the part. Client phones the OEM needing the part and hears one of two answers: Part no longer available = 'plane grounded, or make to order = $$$$ and long lead time.
I don't know Cessna's costing formula, but using the example of a shimmy damper, if they don't have one in stock at all, and offer to make it special order, I can imagine that beginning with having to locate very old tooling for forging, and sending it out for a subcontractor to interrupt their production of something else, to set up a machine to forge one, or a few blanks, to be specially machined into parts. The subcontractor sees a RFQ from Cessna, they are not thinking "let's give a deal today"!