.577/450 Martini-Henry question

I have a .577/450 MH with a sycamore plug in the bullet nose - probably not quite visible in the scan but it’s definitely there! I know that these plugs were found in several variants of the .577 Snider round but I can’t find any reference to their use in the .577/450 MH. Any ideas?

I would say almost certainly a commercial loading. As I am sure you know, there was no service approved .450 Martini round with a plugged bullet. As you stated plugged bullets were used in the .577 Snider Marks I to VI and subsequent marks used hollow nosed bullets with the tip spun over the hole.

Probably an Eley made round.