G'day John.
Are you sure the racks were 'in service' and not post-service? They seem the wrong shape and location for boat racks, too far down he sides of the roof. If army mods, then I'd guess they were cam pole (bamboo) holders.
I joined 51FNQR in LAte '89 and we still had the LC for a couple of years. Never had boats at Platoon/ patrol level back then. It was only after we received the Perenties that the patrol allocation of boats came. First 2x 3.8m Quintrex/ patrol on the roof racks for inshore work. Pretty sure these had a Johnston 25 on the back. about12monthls later (IIRC) we received the 4.2m Stacers, 1 per Patrol, on trailers. These had a 40Horse motor. Usually economical, but with a full patrol load and heavy seas, we could have done with 60s! In recent year, the Zodiac rubber-ducky has been supplied.
They are easy to spot at auctions due to addition of lifting lugs low on the floor/ gunwale bracing. They also got a floor when most civvy ones I've seen are bare.