This cell line was developed by the Medical Research Council (UK) from the lung tissue of a Caucasian baby boy, of 14 weeks gestation, aborted in the UK in or before September 1966. His designation stands for Medical Research Council, cell culture 5.
[T]he MRC-5 cell line was developed in September 1966 from lung tissue taken from a fourteen-week-old fetus aborted for psychiatric reasons from a twenty-seven-year-old physically healthy woman.
The cell line was developed by J.P. Jacobs and associates in September 1966. The British researchers wrote:
We have developed another strain of cells, also derived from fetal lung tissue, taken from a 14-week male fetus removed for psychiatric reasons from a 27 year old woman with a genetically normal family history and no sign of neoplastic disease both at abortion and for at least three years afterward… Our studies indicate that by presently accepted criteria, MRC-5 cells—in common with WI-38 cells of similar origin—have normal characteristics and so could be used for the same purposes as WI-38 cells.
The cells were derived from the lungs of a 14-week-old normal male fetus at our laboratories in 1966. The family history was genetically normal; there was no indication of any congenital abnormalities and no sign of neoplastic disease at abortion.
MRC-5 is used in (some) Polio, Rabies, Chickenpox, Hepatitis-A, Zostavax for shingles vaccines.
 Alvin Wong, M.D., The Ethics of HEK 293, National Catholic Bioethics Center (2006), p. 476, in https://www.pdcnet.org/C1257D43006C9AB1/file/5265B61D5497F52585257D94004802BB/$FILE/ncbq_2006_0006_0003_0077_0099.pdf