Agreed, 1952.
In '52, Lister changed how their serial numbers worked; from a straight numerical sequence (possibly followed by some letters/numbers to indicate features such as reverse-rotation, marine use, etc.) to a carefully designed - and yet possibly utterly confusing - system which is:
Engine No | Type | Variant | Year
So yours is engine no. 12491
It's type D
Variant is K
Year is "2" - counted from 1950 (even though it only started in '52) = 1952.
Unfortunately the CS range were indicated as "51","61", "81", or "102", "122" etc. which can make them easy to mis-identify as an older engine...