Interesting topic !

In the meantime, I have researched further and also found a few things about so called Howdah rifles in various caliber.

The word Howdah rifle seems to have been used less, I did not find it in any book about hunting in India in the past, but some hunters had DR made with shorter barrels than usual, which seems logical when you need a handy weapon to shoot at close range from elephant's back. The majority of the DR of the time, especially the much loved DR caliber 500 Express, were very often supplied with 28"barrels.

It is not clear whether there were many such rifles, because the hunt from the Howdah, compared to other types of tiger hunting, was done less common.