This is a high-quality rifle in very good condition.
Johann Nowotny in Prag / Praha was a renowned gunmaker and dealer, meaning: some guns bearing his name may have been made in his own shop, others were bought in from makers in Austria, Germany, Belgium, maybe in finished state, maybe unfinished ("in the white"), maybe as parts.
Finding out who really made what is detective's work.
As I see no proof marks I assume the gun was made and sold before 1892 or so (in this year official proof became mandatory in the Austrian empire).
1880's might also fit well to the breech with Jones underlever.
I do not know the small stamps on the underside of the barrel (that intertwined "TS" or whatever it is might lead somewhere).

One could do a chamber cast and run a soft lead slug through the barrels, to determine exact dimensions and the cartridge.

That small oval inlay is the "nameplate" of an owner, presumably the first owner. The crown indicates he was from nobility, a Graf or Count, with the initials E and N.
As there where quite countless Counts back then and there is no coat of arms, I see no way to go further here.
The number engraved on the buttplate may be an inventary number, so the owner may have been quite wealthy with a household big enough to be accounted for.

The engraved metal strip on the right side of the stock is for keeping a record of the game shot with this rifle. The letters are abbreviations for different game species, and small nails would be driven in for each animal shot, along the faint parallel lines that can be seen leading away from the metal strip.
Why there are no such nails, who knows?
I have seen a couple of guns from Bohemia with this system (and plenty of nails), maybe this was a local fashion ?

Value is a tricky question. I am not knowledgeable enough and anyway it is not possible without holding the gun in hands. Certainly a couple of 1000 Euros or USD, my guess would be a range between 2500 and 5000.
Your best way to find out a value number is to go to an auction house specialised in collector's arms and have it appreciated, or go through auction records to see what comparable pieces were sold for.
Certainly it is worth enough to keep it safe and cared for, and not to try any "renovations". This rifle is in very good condition, probably it is all original. Any re-blueing or stock conditioning (except a rub of the right oil then and now) is unnecessary and takes away value for a collector.

fuhrmann