Rubber Block Plug on 1350?

I've had my powerhead off a 1971 Merc. 1350 and saw what seemed to be half of a damaged rubber block or plug partially closing one of the smaller elongated ports when looking at the powerhead from the bottom. Forgot if it was on the exhaust or head cover side? Looks to be the remains of a molded black plug about 2 inches wide and an inch deep by 1/4 tall. Should this plug be replaced with a new one and was the intention to restrict water (if on the head side) or exhaust to some degree? I've heard some talk about a wall of water cooling the head and maybe this plug was there to help with that? Can't find it in my manual anywhere and don't know what to order. What is the result of running without the plug? Thanks.

Re: Rubber Block Plug on 1350?

Get on the crowleymarine.com web site and look up the exploded parts list. There is a rubber seal around the cooling water tube, where it sits in the exhaust extension. The water tube on my '77 Merc 1500 was partially crushed by salt, and I ended up replacing te tube and seal. That was the only rubber thing I saw in the vicinity.