This button resembles a Royal Navy officers' button (1843-1891).
However, the rope goes the other way, the gilding is below par and it has a floppy shank.
The back mark is Extra Rich Quality and its size is 19 mm.
I have another, near identical one: different front die, unmarked steel back, floppy shank.
Buttons for boys' sailor suits are usually lower quality and exact copies of officers' buttons.
Can I have your opinion on these?
PS I found one for sale on ebay. It has a 3 crown backmark:
https://www.ebay.co.uk/itm/19th-C-Vi...YAAOSwNWxaR7OB