I would guess it is for mounting some type of sight. Maybe a peep sight of some type but not sure. I assume you are talking about holes like these.

 

Yes, the two guns I have are like that one. Two holes with no threads, just holes.
Do goose hunters still shoot 36"ers barrels anymore?
Never shot one yet, should be fun.

 

Shorten the barrel, D/T the receiver and you have a dandy smooth bore slug gun.

 

I would just buy a slug gun before I would chop up and drill a hole gun that was passed down to me. I would like to have one to see how they would do at the community turkey shoots. I bet if you get the load right with that long barrel they could throw a tight pattern at a 25 yard turkey shoot. :biggrin:

 

Probably closer to 40 yards, there the old Goose Guns.

 

I would bet they would shoot tight patterns to at least 40 yds or better. I have an old humpback a-5 with a 36 inch barrel and full fixed choke. It is good to about 50 yds on my turkey targets........

 

The holy grail buckshot gun around here is the Marlin Swamp Gun, same model 55 shotgun with a factory 18 inch barrel, would be tempted to chop and choke one of them.

 

AUUUUUGHHH!!!
If it's origenal, don't touch it!!!
Go get a Mossberg shorty cheap or a long barrel cheaper and just saw the danged thing off.

The Goose guns are unbelievable fun guns.... I bought 2 and have less than $100 in 'em both.
Refinished the stocks and they're like new.

Friends laughed as I backed out with one at a dove hunt...and kept backing out, and out, and out.
The long barrel assures that you will follow through on your swing.
I load 1 3/8 oz #6 shot pushed by a healthy dump of Unique...
On the level, 60 yards shots are no brainers if you're on your game, but finding birds can be a challenge.
Late morning white wings at nose bleed altitude coming in from Mexico are no issue, once you get the HUGE lead dialed in.
They take a LONG time to fall.

 

I have a 55S slug gun the two holes on mine are tapped and plugged with screws, figured they were for some sort of sight.

 

I can answer part of your question. I am an avid goose hunter...no, I have never seen anyone who hunts any waterfowl seriously use one of those.

 

I've got a 55 slug gun. There was an aperture receiver sight available for the slug gun models. I imagine the holes were drilled in all the receivers and maybe only tapped in the rifle sighted models (or your the threads are plugged with gunk). The parts for the sight are listed at https://www.gunpartscorp.com/Manufacturers/MarlinGlenfield-33379/Shotguns-37334/55-41105.htm?page=4