1851 colt?

If you're referring to the OP it's not a fantasy navy it's a fantasy Griswold & Gunnison. The 1860 barrel is entirely different from the navy and the G&G.
Yep the Army is round all the way back. About 8 years ago EMF had G&G’s on sale. I bought one. The “ receiver” back portion of the barrel is half octagon with clean flats and squared edges. I do believe the grip frame and grips are slightly smaller too.

A sample of the four.
The key mechanical difference of the 1860, 61, and 62 is the "creeping loading lever" as an improvement over the simple pivot screw* of earlier models. I don't know how much better it works, if any.

*A classmate cast balls out of hard scrap lead and managed to break the pivot screw of his 1851 repro. He paid a gunsmith to extract the stub and replace the screw, and promptly traded it in on a breechloader. We are talking a $7.50 repair and a $37.50 trade in on a K38, I think $87.50 at the time.
What makes it an 1851 Navy in a fantasy 44 caliber and not an 1860 Army with a brass frame. ??

Most notably, it is modeled more like the 1851 than the 1860.
It doesn't have the geared loading lever of the 1860 or the smooth rounded transition from barrel to frame of the 1860.
And the 1860 wasn't offered with a brass frame so it would still be a "fantasy" as some like to call it even if you wanted to call it a brass frame 1860.
Plus the front sight implies 1851..