In my .357 Magnum Speed die post I pondered what made the .357 mag speed die specific to .357 magnum. I fathomed that it was only the decap flare stem that made it .357 specific but alas I was wrong! In fact not only is the decap flare stem different, the die body itself has different INTERNAL dimensions...Looks like just over 1/8" which is how much longer the .357 is to the .38 special. But the bullet seater stem is longer in the .38 special set as well, which makes sense since it would have to reach further down into the die to seat a bullet.
Picture of compared parts coming soon, If you are looking at getting one of these speed dies to load both 38 special and .357 mag I'm sorry but it will not work. The dies are very cartridge specific as the the case lengths are different .135" difference actually. The whack a mole type lee loader will work for both .357 magnum as well as .38 Special as will most 3 or 4 die sets. Would you rather have 2 dies to load 2 cartridges or 3 dies to load two cartridges when those 3 dies are still in production and widely available.
Sorry for any inconvenience I may have caused on this assumption of interchangeability. I myself would have liked to see one speed die be able to load both as that would have made a very handy on the range set up for testing. Stay safe out there.