The difference is in the hub, not the wheel disks. In the 1956 SNL the section view shows a hub as on the left, but the exploded view shows a hub as on the right. They have the same part number, so as far as the engineering was concerned, they were functionally interchangeable. It appears that the one on the left has a cast or forged shape and the one on the right is simpler turned cone.
KL
Thanks Kurt. That drawing is very helpful. I was looking thru my stash and I think the RB Models metal barrel for the 17cm Matterhorn should work as a replacement. Just need to do a little sanding to create the flare.
There is a picture of the M53 and M55, side by side, on the tank-afv.com website.
To me, the gun barrels look very different.
I could have sworn I sent this from a donut shop a while ago, but I forgot to hit the “reply” button:
I had said simply “Chuck it up” because if you don’t have a lathe you can use a drill press or just an electric drill mounted to your workbench. If you use a drill press just go cautiously as the bearings aren’t designed for the lateral stress. One small job won’t hurt it - I’ve used my press as a mill before.
By the way, those funky cones in your photo look like they originally planned another part to go there - you can see the outline. If you want to replicate the two styles you could cut down some hubs from other kits and add them. Add a grease nipple and you’re good.
My own walk arounds don’t show this difference. The idler hub is obviously beefier due to those lateral stresses I mentioned above.
The M53 blog at Track-Link unfortunately seems to stall; FWIW, my build report of the Renwal/Revell M55 kit can be sen here: Build report - US M55 203 mm Panzerhaubitze .
As for the site of the M53 linked by Gino, its photos are definitely showing the prototype:
Starting with the first pic, the MG mount mentioned in the caption was not on the roof of the production vehicles, but mounted to the rotating commander’s cupola, as can be seen in the TM illustration that’s shown below this.
In the next two photos, the vents above the drive sprockets lead to the armored final drive covers. Behind the mesh screen, there are the engine and APU exhaust outlets on the right hand side only, nothing on the left (strange, but). The driver’s and, on the right, gunner’s doors had splash rails around their openings on production vehicles.
It’s a crying shame that ILK didn’t give a turret interior - but I can understand them!
Peter
Making some progress on the M53. Here is my effort on a new barrel. I also made a new barrel sleeve as the kit part is overly simplified, to be kind. I used an RB Models German 17cm barrel as basis. I made a 60 mile road trip to visit an M55 in Maybee, Mi. That vehicle has the two different style road wheel hubs shown below. Looks like ILK intended to give the options, but forgot the hub cap on the one. It’s too bad they fumbled the ball regards the gun on this one, as the detail on the rest of the kit isn’t bad. I mean after all, it’s an artillery piece and the cannon is the main attraction.
Nice recovery on the barrel. It came out looking great.
Those two M53’s you posted identify as M55’s 
Image below shows an M53 (foreground) and an M55 (background)
Angled gun travel lock on the M55, straight on the M53.
Gun on M53 extends way forward of the travel lock at the front of the chassis
On the M55 the muzzle extends over the front by a few inches and the travel lock is angled backwards to avoid slipping off the muzzle.
More info here:
http://afvdb.50megs.com/usa/pics/155mmspgm53/155mmspgm53.html
Edited the first two sentences above to clarify the message.
Stumbled across a turned aluminum replacement barrel, sleeve, rounds for the I 
Kit M53 on eBay from a company I am not familiar with. It looks pretty nice.
I’ve gotten several barrels from Chuanyu. They are well made and accurate.
From the OP:
That’s the one the OP ordered. It’s the one that started the whole discussion to begin with. It seems someone needs to bust out the calipers. Anyone?
Thanks Gino.
To my eyes, ammunition storage inside the vehicles, gun barrels, and travel locks look different.
That as well, hard to see the ammo storage if the rear doors are closed (gate guards, open air museums et.c).
I edited my post to clarify that “those two” referred to the ones posted by Dan,
not the M53 and M55 in the B&W photo in my post.
Sorry for being unclear
8 inch → 5.80 mm in 1/35
155 mm → 4.43 mm in 1/35
31% increase from 155 mm to 8 inch.
Well yes, but does anyone actually have measurements of the real barrel, including the length? Maybe the one from Golden, CO? 
Then we could compare those to the kit barrel, which still needs to be measured in spite of your excellent math skills. 
The total visible length of the barrel outside the turret?
Needs a TM or a real M53 that somone can climb on.
Measuring from photos is not always an exact science …
Btw: measuring the length of the real barrel is not a caliper job, thats more like a tape measure task …
Thank you! 
Original photo is here
The photo above should provide a reasonably “close enough” “measurement” for the visible length of the barrel by comparing with the roadwheels.
Approximately from the center of the first roadwheel to the front edge of the “hubcap” of the sixth roadwheel, red lines in the image above.
We have from Hunnicutt that the M53 has a 71" barrel overhang in the travel position. That is the number I used to estimate the barrel length. Ideally I like to work to actual numbers, but for now this will do. At least when the M55 comes out, I have some actual measurements.
