just Googled Dunlop Sp41 tyres.....
Longstonetyres says (
http://www.longstonetyres.co.uk/page/dunlop-sp41)
The Dunlop SP41 was Dunlops first commercial venture into marketing the new radial tyre technology. The SP41 was sold alongside the C41 crossply tyre from i believe 1963 until 1968. In 1968 the Dunlop SP Sport tyre became the most prominent radial tyre from Dunlop. I'm afraid i cannot confirm these dates exactly but from these period fitment guides it appears that the SP Sport takes over in 1968.So if you have 5 tyres the same, with little wear then looks like original miles. The tyres may also have a manufacture date moulding, like the modern DOT number.
Plug leads were probably 1/4 inch (i.e. 6mm), like a 1975 (other brand) thing I have in the garage. Leads have become thicker over the years.
This is not a barn find, it's a time capsule.