Concur with posts #550-554. The airline I worked for always had at least one "tech rep" based at the manufacturer (regardless of which manufacturer) when "our" new a/c were going down the line - start to finish.
In addition, it MAY not apply to long-standing 737 operators, but
AFAIK all manufacturers have/had specially-trained tech reps who were based with the customer for the 1st year or so of operating a new type. I guess not applicable to Alaska AL or United, but for example, in cases of checking the plug of a/c already in service, that tech rep would have been very hands-on with supervising the customer's own staff doing those checks.