His first choice was a Randall, but they wouldn’t put him to the top of the wait list.
The Randall story is that Stallone wanted to be moved up the Randall list and comped a few knives. Randall passed on the deal. They were looking at either some of the larger bowies, the #18, or a totally new model based on those knives. The fact they were interested in Randall knives is backed up in an early 1980's Soldier of Fortune article/interview with people from the movie.
Bill Moran was also offered a similar deal. He passed too.
SoF 6 / 1985 -
"Maffatone wanted Stallone to choose another knife as Rambo's trustiest weapon rather than reappear with a Jimmy Lile creation given the situation and terrain of the mission. "I felt it was strictly for the cameras," commented Maffatone.
"I would certainly have carried something different if I expected to be using it on human targets a great deal. But Sly is a genius when it comes to how things look on film." How Jimmy Lile blades look - as anyone who saw First Blood can testify - is huge, intimidating and deadly.
Stallone claims the choice was mostly serendipity.
"I was going to use the Randall knife which is very similar," he recalled.
"Then I ran into Jimmy Lile in a knife store and we decided maybe we could come up with something a little more modern. You know, with the compass and the screwdrivers in both ends? I was also thinking about using a Randall Bowie, but on film the Bowie looks so huge."
No matter. It's Stallone's moves and the way Maffatone taught him to attack an adversary that make the one-on-one encounters with both NVA and Russian advisers easy for genuine combat veterans to swallow."
----------------------
Article on Tony Maffatone, Stallone's adviser. it has a good bit on knives, and you can see that they knew about Moran -
https://thomaspluck.com/2008/09/04/a-tribute-to-tony-maffatone/ .