Dr. William Hunter began to collect coins in about 1770, spending over £22,000. By the time of his death in 1783 he had 12,000 ancient Greek coins and a similar quantity of Roman coins.
Although hemidrachms of this denomination are common in museum collections, this tetradrachm is unique. Nothing like it has shown up in 300 years.
This unique coin was first published by M. L. Dutens in 1776. George Macdonald assigned the tetradrachm to 200-133 BC – the period before the start of the Cistophoric-weight tetradrachms. Barclay Head dated it post-190 BC. Presumably he chose 190 BC to place the series after the Battle of Magnesia. This dating is odd because the hemidrachms were dated to 400-300 BC. Both Head and Macdonald thought this was 100 years later.
Perhaps MacDonald couldn’t date it to nearer to the hemidrachms because of the weight standard. The weight, 13.58g, is low for a tetradrachm. MacDonald felt the weight standard was 'Rhodian'. He called the gorgon/cow hemidrachm weight standard 'Persic'.
The HR monogram was used on Macedonian Artemis/club tetradrachms (167-149 BC), Macedonian Pan/Athena tetradrachms (277-239 BC), Macedonian prow/Poseidon tetrachms (294-288 BC), Babylon mint Athena/Nike gold staters (336-323 BC), Alexander Balas tetradrachms (Tyre mint, 150-145 BC), and bronzes of Kyme and Seleucis and Pieria. None of those issues seems possible to link to this tetradrachm.
It is possible that this is an 18th century fantasy. It would be worthwhile to look at all of the unique coins in Hunter's collection to see if they have any traits in common. Of all the coins in all the world's museums, this tetradrachm is the one I'd most like to inspect in person.