The barrel should actually be measured from its rear to its front. That's only a couple thousandths of an inch shorter than the measurement to the front of the cylinder, so you're close enough in this instance. I clicked on your photo to enlarge it, and that's definitely not a nominal 4 3/4" barrel. Looks like a special order 5" to me. That serial number, if it is DF31, would make the gun a very early Field Grade gun. Or did you mean that it is DF31XX (4 digits)?
It's a weird proposition to me that someone would pay the upcharge for only an extra 1/4" barrel, particularly when the FG's are usually purchased to save money. That makes me wonder if the barrel was originally longer, and was subsequently shortened by the factory, or someone else.
If you're curious enough, it might be worth a call to Freedom Arms, but you MIGHT have to pay for the info.
Another thing that strikes me is that the very early FG's had an aluminum adapter base and used Ruger rear sights, but your rear sight is not a Ruger. Of course, it could have been changed out, or perhaps the change was made to the FA rear sight sooner than I knew of. On the other hand if the serial number is 4 digits and not 2, then the question is answered: the change was made by then.
With regard to your front sight question, the standard heights are in .050" increments: like .330, .380, .430, .480" etc. What you have looks like the standard FA sight. Perhaps your measurement is off a bit, or their tolerances were a bit sloppy on that one. I usually see them within a couple thousandths.
