Gerben, I think you made a typo. Isn't the manual date 5/1936? In the data I have compiled there is an example in the 398000 SN range that has the curved grip. There is also one lower than 400000 that had the grip replaced with a curved grip as I had recorded it earlier with a straight grip. There is often some overlap in changed features during Mauser production. It is difficult if not impossible to precisely state when a change occurred. It is a very nice example.
Regards,