We know that the HJ was introduced in 1933 (HJ Wiki) but as far as I know, it has not been determined why it ended up looking like it did. Looking at pictures of trench knives from WWI today I came across a knife by WK&C (pic 1). It has details that appear on the early Ernst Pack u. Söhne knives (pic 2) and also the later standardised HJ knife (pic 3 - early Tiger).

wkc.jpg pak.jpg wkc1.jpg

Just a theory of course but it makes you wonder whether it was WK&C who perhaps came up with the first design of the HJ knife that was later adopted by the RJF.