A sovereign is a gold coin which traditionally had the value of £1 sterling.
There is no mention of what Robert Browning, or Robert, is slang for in any of the dictionaries named below.
Robert Browning, the poet, famously used the word twat in a poem entitled Pippa Passes, mistakenly thinking that it was part of a nun's habit (not that this throws any light on the present question).
Dictionary of Victorian sexual slang
Dictionary of the Vulgar Tongue (1811)
Cockney Rhyming Slang
Taboo Cockney Rhyming Slang
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