The grave reputed to contain the remains of George III's granddaughter has been secretly restored.
Experts say Caroline Prytherch and her sister were related to the former king due to his links with Hannah Lightfoot during his time as Prince of Wales.
The previously lost family grave of Ms Prytherch at St David's Church cemetery near Carmarthen had been split in two because of damage and overgrown ivy.
Her sister Charlotte's nearby tomb was restored in 2000.
The association of George III, whose reign of king was between 1760 and 1820, and Ms Lightfoot has been widely reported for more than 250 years.
It is believed they married in secrecy in London in 1759 and had three children. But in 1761, without the marriage being dissolved, the now king married Charlotte Sophia of Mecklenburg-Strelitz who became his queen.
One of George III's and Hannah's daughters, Catherine Augusta, married Carmarthen doctor James Dalton and had four children including two daughters Charlotte and Caroline.
The king once gifted an 18th Century piped organ, originally planned for the Chapel Royal in Windsor, to St Peter's Church because of his links with Carmarthen.
St David's cemetery is the final resting place of his granddaughter, her husband Daniel and three of their children - and now the grave has undergone a "secret restoration" following decades of neglect.
Cranes will lift the one-tonne lid of Ms Prytherch's box grave back into position in a refurbishment by the Thomas and Elizabeth Mayhook Charity - which owns the cemetery - that has taken several weeks.
"There are so many unanswered questions and unexplained events over the Hannah Lightfoot story," said Richard Goodridge, the charity chairman.
"But what is remarkable from a local perspective is that Caroline's youngest daughter Margaret who died aged eight in 1839, and Caroline's elder sister Charlotte, who died aged 27 in 1832, are the only two persons buried in a brick vaulted tomb in the centre of the chancel directly in front of the altar of St Peter's Church in Carmarthen.
"These two young ladies were regarded as extremely important but nobody can say why. One can only imagine, which is why there has been so much intrigue and fascination about this family over the centuries as to their alleged Royal grandfather George III."
Restoration of the ornate Coalbrookdale cast iron railings that surround the tomb of Ms Prytherch, who died aged 70 in 1875, will take place later this year once funding is secured.