There wasn't one, a pro 2nd side that is, at the Supreme Court. It was successfully argued at the district court level, during pretrial motions. The judge dismissed the case on the basis that no violation of a valid law was alleged.
Only the government was represented before the Supreme Court. The NRA did not file a friend of the Court brief, being mostly a target shooting and hunting group in those days, the persons originally charged were also not represented. Today that would not happen. Even if they could not be located, which was more or less the case then, counsel would be appointed to represent them. One of the parties was dead by the time the Court rendered its opinion.
See this collection of Miller Documents for the whole sordid story.
"Today that would not happen."
This certainly isn't our grandfathers NRA, not any time soon will a major 2nd amend issue go unchallenged by our side.