My 1970 131 did not have headrests on what I believe to be the original seats.
What I hated was the Betty Swollocks on a long summer trip so investigated a conversion to leather. Was quoted £1800 for a re-upholster of the existing seats.
Spotted a complete cream leather interior from an S40 on ebay, measured a neighbours car and reckoned they would fit and got the lot for £100.
Fronts went in easily with four new holes in the floor and a couple of brackets under the back end of the runners. Wired in the electrics for the drivers seat.
The back required a lot of cutting and trimming to fit but went in eventually.
The originals are in the loft in case a new owner wanted to refit them.
I reckon it looks pretty good and is much more comfortable.
I know the purists will not like it but it makes the car much more usable.
Brian