The 1863 was a "rifled musket."
If it is a breach loader it is NOT an original 1863. If its markings are appropriate for an 1863 it is a conversion. There was more then one type of conversion, but the most I have seen were Allin. Probably because I believe it was officially adopted by the Army.
The breach block does open forward like a trapdoor doesn't it? There was also a conversion that opened from the side.
Just because it isn't an "original" 1863 doesn't mean it doesn't have any value. The conversions are collectable in their own right.
If you get a chance, stick a ruler over the bore. Just outta curiosity I'd like to know if its diameter is .50 or .58. If it is a .58 then I should think it would be the 1st Allin Conversion of 1861 Springfields started in 1864 as I remember.
Wait a minute! I know what youse guys (my yankee accent) is thinking! No, I don't remember because I was there!
I seem to remember from my studies when I was satisfying an interest in conversions!
Anyway, the improved, 50-70, 2d Allen conversion was in what... 1866?
OH! How many barrel bands!?