Sinn Féin has received another large donation of almost £500,000 from the same deceased Englishman who already left them £1.5 million.
Mr Hampton, a former market trader, died in 2018 in Pembrokeshire in Wales.
Combined, the almost £2m windfall is the largest ever known donation to a political party in Northern Ireland from a single donor.
Informed sources told BBC News NI the additional half-a-million has already been paid to the party.
It will be recorded in an updated version of the Electoral Commission's list of political donations due to be published at the end of the month.
When details of Mr Hampton's donations emerged, questions arose about his mental health.
It is believed he had a psychiatric history. Many years ago, he spent time in a psychiatric unit after he mutilated part of his genitals.
His father, Tim Hampton, was a wealthy businessman who had significant commercial interests in the village of Fenstanton in Cambridgeshire.
'He will be laughing in his grave'
According to Billy Hampton's friends, he was unhappy during his life because he could not access all the inheritance he felt he was entitled in a single lump sum.
As a result, friend Dave Morton said Mr Hampton decided to leave his own fortune to Sinn Féin "out of spite" and "to say 'up you' to the British establishment."
"He had a really wicked sense of humour and he would have thought that was funny," Mr Morton said.
"Basically no one helped him, and he got it into his head that the establishment was out to get him.
"He felt round every corner someone was watching him which they weren't.
"It was probably all in his head. He will be laughing in his grave now.
"He never even mentioned Irish politics. All he said was when he was over in Ireland he generally had a good time."
Up until his death he had led a nomadic existence living in a camper van and travelling around the UK, Europe and further afield.
Mr Hampton made his will in 1997 when he was living in a caravan in County Cavan.
According to a copy of it, the executors and trustees were former IRA chief-of-staff Joe Cahill and another republican called Dessie Mackin.
In one letter written in France in 2001, Mr Hampton wrote: "I am much less paranoid than normal, and do not suffer from a persecution complex at all here in France."
In the same letter, written four years after he wrote his will, he said: "Sinn Féin will not speak to me now for security reasons."
The party has erected a memorial stone in his honour at a cemetery in Hannahstown, Co Antrim.
The inscription reads: "True friend of Ireland. Remembered by his true friends and comrades in Sinn Féin."
A spokesperson for the Electoral Commission said: "We will be publishing donations reported by political parties in Q3 [quarter three] 2019 on 26 November.
"As such it would not be appropriate for us to comment on this before then.
"There is no limit on the size of donations a party can accept. However, donations over £7,500 (or smaller donations from the same donor that add together to exceed that threshold) must be reported to us."
According the Probate Registry of Wales, Mr Hampton's will was worth £2.6m at the time of his death.
As Sinn Féin is the main beneficiary, the party could receive further instalments of money in the future.