When my son was in the SF dive school in Key West, I wanted a special graduation present for him. We had a gun and knife show in Ft Walton Beach. I promoted the show for years, so dropped by to see the Randall dealer that always came to the show. Told him I wanted a Randall Dive Knife, he reached under the table, pulled out and handed me a Randal with a leather sheath. So I said I think it should have a nylon sheath, he said it will work and if not your son can have a nylon sheath made. Ok, so I walk off after spending a pile of US dollars. Randall’s are just expensive. But as the dealer said to me, anywhere you go with a Randall you are well dressed. That was near 30 years ago. So a month or so ago my son calls me, do you remember my dive knife you bought me, of course I do. You know what that knife is, no, it is one of six pre production samples Randall made, that never went into production. Ah, so that is why it had a leather sheath then, it is really a dive knife not intended to be used as a dive knife. Yep.
My most expensive knife I do not own.
I am not sure about the knife being a pre- production diving knife but Randall has been making the model 16 diving knife for a long time, probably back into the 1960's. It has been made with several different blade designs and handle combinations. It used to be supplied standard with the wax impregnated leather sheath and then with an option of the nylon sheath as far back as the 1980's. I have had this one with the saw teeth for a few years. I got it with both the waxed leather and the nylon sheaths. I dive with it on occasion as well as the Spyderco in the picture. I think what you may be describing is a Model 16 Special Fighter. I had one of them back in the 1980's as well.
A Standard Model 16 looks pretty much like this.
And a Model 16 Special Fighter looks pretty much like this.
Handles, fingerguards and sheaths can be many different types.