I was using a transcription of vital records that indicated a relative was named Jas. I assumed he was James and was searching for that name. Turns out the record that was transcribed had used "Jos." for Joseph and the transcriber made the "o" an "a."
So copy down what the transcriber wrote, but keep in mind that they might have incorrectly copied the record--or that the record was difficult to read and that the transcriber did the best they could.