all of modern akg's are still ''diffused-field'', even the k701 and Q701 are but in a different manner and not the same way the 240DF's were. 240DF was originally made to fellow a special standard for radio broadcasting and was done bit differently. i don't have any info on them besides the ones i scraped from the internet since trying to speak with akg's representative and get info of them was impossible. they just kept suggesting the k702,k702,k702 and ignoring my question so the DF's will always be a mystery. only other thing i know is the HD800's from sennheiser was gonna fellow same IRT standards as the DF's but was strongly recommended not to and sennheiser developed their own way for ''diffused-field'' equalization. i think the beyer DT48S fellows the same IRT standard as the DF's but it's near impossible to find them and only few people i read from the internet says they're very good headphone and very neutral like the DF's.
241 and 250 is interesting but if they're indeed identical to the sextetts then going by logic and assumption, i would think they're free-field instead. it can be possible the 241 and 250 was an experimented diffused-field headphone but i guess they really didn't fully introduce ''diffused-field'' till the DF's,which gave it it's name cause it was one of the first mass marketed diffused-field headphone. who knows. only thing i know is, older akg's sound darn good