That is good money for a fake
That is a simmons conversion is it not. If I was buying one today they would have to let me take it apart or no sale
They say there are more fakes now that originals were ever made
Some good reading on them here in this post not my words
Standard barrel lenghts for the 28ga, were 26 or 28 inches, the fact that this one measures 26 & 1/4 would be in the tolerance range for the barrel. Based on your description of the vent rib posts, it sounds like the STANDARD Vent Rib by Simmons. If you look on the side of the rib just above where you see Winchester Model 12 on the barrel you should see Simmons Gun Specialities on the side of the rib. Note; Simmons reblues all Md 12s after the rib is brazed on, and the receiver is machined for the rib ramp. Skeet guns came with checkered stocks, but without seeing the checkering pattern, I couldn't tell you if they are factory, duplicates or true aftermarket. Engine turned bolt and carriers is common on factory Skeet guns, but not often found on plain barrel guns.THIS IS IMPORTANT the 28ga. was built on the 20ga. frame just like the 16 ga. was. Remove the butt stock and look at the back of the receiver were the stock goes, just above the receiver shank, (that's where the stock bolt screws into the receiver) you should see a 28 stamped into the metal, just above the groove where the stock lip fits. Simmons use to convert 16ga and 20ga guns into 28ga. guns, this included rerolling the barrel and re roll die stamping of all the Winchester info on the barrel.
I would be very careful here, If the serial number of the gun falls below serial number 720137 (1937 start date) or above serial number 1770398 (the last year the 28ga was made in production) BE leary!!!
Simmons conversions range between $1200-$2000. A true 28ga. with a Simmons rib, $1500-$3000.
Regards Dave
A couple of other places to look is the magazine band should have a small 28 stamped on it, it will be on the back side left or the front side right. The action slide rod (the rod that goes into the receiver) should also have a 28 on the inside. you may have to remove the barrel assy. from the receiver to see it.