IMHO the standard of the Howdah's is the Wilkinson. Made in .577 Snider rifle cartridge caliber. I have thought about this project many times.
The first Howdah you showed was a 20 bore, sawhandle,smooth barrelled gun. I believe that one to be made up.
I would buy and old beater hammer gun, preferably a 28 bore, from one of the English auction houses. There are plenty there, and if the gun is out of proof, they go cheap. Then I would sleeve it to the .577 cartridge. Soemetimes the stock is even usable with alteration. Put a lanyard in the handle and yoyur ready to go.
Regulation is not necessary as by definition these were used in the Howdah when the tiger was coming in to kill you.
The Pedersoli is out for me. It must be English - period.
Good luck
JZ