The explanations above should sort out the confusion. Blue Grass was a trademarked name for a line of goods Belknap offered and then the name was purchased after they closed, promoted to brand status with former Belknap line-mates such as Primble & India Steel also purchased by the same group and becoming lines under Blue Grass. The same way Genesis was was a car made by Hyundai but suddenly got promoted to 'maker' so now you have Genesis cars with multiple models.
Belknap contracted their knives from many big makers over the years and never relied solely on one supplier at a time. To make things a little blurrier they also switched manufacturers of knives without changing the model number. I collect Schrades so I wanted to find a Primble 5371 stockman made by Schrade (well, technically it was made by Ulster but no need to muddy the waters further.) The problem is I'm aware of at least 3 manufacturers who made the knife for Belknap under that model number and at a fast glance they all look the same. Hold them together and you'll spot the differences.
Belknap's catalogs help sort it out a little because they always sold the lines of their suppliers alongside their own lines. Open a catalog from the '50s or '60s and in the cutlery section there are Primbles and Schrades and Bokers, etc. Sometimes you can figure out the manufacturer of a Primble or Blue Grass by matching it up to manufacturer's knives on the following pages. Sometimes you can't. The Winchester angle was a red herring - they have no history with Belknap. I think Remington was likely who the seller was trying to think of? In the '20s-'30s they were by far the biggest supplier of knives to Belknap, but your knife isn't that old.
The 5721 Barlow is found in the 1937 and 1950 catalogs BUT it's shaped a little different:
I think what you've got there is actually a 1950s 5724:
In the '50s the two big suppliers were Schrade and Boker. Wostenholm was a minor supplier as well. That barlow you got there ain't a Boker or a Wostenholm. I'm pretty sure that was supplied by Schrade - but again, actually made by Ulster...oh fudge, the waters got muddied again.
Congrats on finding a nice old Ulster barlow.