The relevant portion of the rules would appear to be:
9 FAM 402.2-5(E)(1) (U) Commercial or Industrial Workers
(CT:VISA-1; 11-18-2015)
a. (U) An alien coming to the United States to install, service, or repair commercial or industrial equipment or machinery purchased from a company outside the United States or to train U.S. workers to perform such services. However, in such cases, the contract of sale must specifically require the seller to provide such services or training and the visa applicant must possess specialized knowledge essential to the seller’s contractual obligation to perform the services or training and must receive no remuneration from a U.S. source.
So providing you are performing service engineer type duties that can only be carried out by an employee of your company, on equipment sold from ourside the US, this would seem to be covered.
It would be wise for future trips to get some documentation covering all points above – e.g. see if your sales contract says an employee of your company must carry out service/repair work, for example.
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