Going to look at a 1954 model 12 Super Speed X in really nice condition. Are there any special things to check on it and what would be a fair value?

Hmm..these are fairy simple and robust mechanicaly...if the takedown mechanism is a bit loose(receiver/barrel joint)..it's fairly easy to adjust for a good fit.

Have no idea what a Model 12 is worth??

That would be a Model 12 Magnum or duck gun chambered for 3'' shells. Also has a bit thicker barrel then standard Model 12s. The buttstock is also shorter & has a lead weight inside it to help with the balance. Should have a red Winchester recoil pad. Can be found with 30 or 32'' barrels.

My only concern would be that you may not be able to shoot steel shot if it has a full choke. I don't know if it would eventually damage the barrel or not, but as I understand it, a modified barrel is much better for steel shot.

I had to go through many guns to find a couple of "good" model 12 heavy duck guns. Most were shot a lot, and have the receivers and bolts battered through years of use with heavy lead loads. Be cautions of them- even ones that may be 90% or better on the outside, may have a good bit of wear on the inside. A quick look and feel up in the bolt slot area of the receiver will tell quickly.

If you do get a good one, it can be shot with some of the Fiocchi Tundra shells, which are sintered tungsten and easy on regular steel barrels. Of course, you may be laughed out of a duck blind for not showing up with a whiz-bang ultra-primo Italian self-loader, and instead a tried and true piece of good old American steel.

I have used one box of the Fiocchi Tundra in an 1897 Winchester, and it worked. It's the most expensive shell I've ever bought, but I wanted to kill some geese with it again- and it worked!

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Lost it all in a boat accident

The Super X & Super Speed (bbl?) markings would make sense for the 3" chambered Model 12 Magnum Duck Gun.

By '54 or so, I think it was only offered in the 30" bbl length. The 32" being discontinued by then. Full choke only.
They came w/a wooden Winchester marked 'Duck Plug' installed in the magazine tube.

But Winchester would accomodate most any special order request if they could. If a 32" bbl length was still in stock in '54 and available for fitting, it could very well have been made up to order.

They are Heavy as the name says. Just under 9lbs usually with that lead weight under the Winchester pad that Davwingman made note of.

Plain, solid rib and vent rib bbls were all still available at the time.

Special order was a straight grip butt stock. Of course it could be ordered in high grade versions too.

Standard M12 stuff when looking at one. Matching serial numbers on the frame and bbl extension. Look carefully for any hint of a re-blue or stock refinish. Check the TD adjustment , make sure it hasn't been taken up so far over the years that there is little adjustment left in it.
They used to offer different bbl sleeves (1 thru 4 I think) for the 12 and 42 to replace the one numbered below it and allow more take-up and make the TD joint tight again. If you can find them they are about $50+ each from parts dealers now and not much of a choice in number size when you do find them.

Solid and vent rib guns should have the Winchester Proof mark on the left side of the bbl breech just to the side of the rib. Also marked on the receiver.
Proofing was always done after the bluing,,so the stamp was punched 'through' the bluing and should show it.

There are aftermarket solid and vent ribs added to many of these guns to enhance values. Proof marks added afterwards. Upgrading of wood, checkering, factory pattern engraving, ect.
Look very closely if it's anything more than a plain jane gun. Even then look for signs of repair/ refinish.

These were factory blued using hot salt bluing technique (after '39). Du-Lite bluing salts were used and still are by most shops and factorys. It's the replication of the polish underneath the color that most of the upgraders can't quite make the grade.

An original condition Heavy Duck M12 is not a common shotgun. One in excellent Original condition even less so.
Most have seen some honest & some hard use. Many have been refinished and some w/upgrading enhancements.

I wouldn't shoot any steel loads out of these guns. The choke is just too tight IMO. A bulged bbl at the choke can result. Salvaging the bbl by cutting leaves you with a 26" cylinder bore Duck Gun. Not a real hot moving item on the resale market.
Maybe choke tubes could be put in and get around any problem,,but I wouldn't want to do anything to a prime old Winchester like these.

An original in excellent condition (Field grade, plain bbl (30") is probably going to be retail priced in the $1200+ market depending on the area. A 32" bbl will get more money. Internet sales will get more because of their exposure generally.

Check the closed auctions on GunBroker and such for values of those actually sold or even those that did not make reserve.
It gives you a good idea what people will pay. Note many of the guns are reblued, restocked or otherwise not original.

Add in a solid or vent rib (original factory 'Pigeon Grade'), Straight grip stock, extension forend, with the excl'nt condition and the prices can go quickly north of $4K. But that is for prime collectors grade shotguns,,not shooters. The value drops quickly when bluing wear is apparent, wood finish is worn or scratched, ect. Just like any other firearm.
M12 collectors have a big field to choose from and can usually be quite picky. It gives people who just want a M12 in nice condition to shoot a chance to buy some very nice condition guns at decent prices.

Thanks to all, some great info. It is a 30" barrel solid rib. Going to look at it this evening.