I saw the episode last night. I thought it was an Ithica Model 37 "stakeout" shotgun. I used to see them advertised in Shotgun News by some of the Law Enforcement distributors.
I have to admit, I get a kick out of it myself. I grew up in S. Florida and tried never to miss an episode. Yes, it is an Ithaca Stakeout. Tubbs' primary HG is a Bodyguard snub.
i used to watch it during he original series run, that musta been an early show. during the later seasons he was carrying,under his sportcoat, a wilson (not that wilson) witness protection with the folding verticle forearm and 3 round mag.