No, the DSE-103 is thicker than the M series.
Well, this is partially untrue...
I found that the 103 carts will fit onto the 401, but if you push them down to hard you end up with a flood of liquid. To get around this issue, you can pop out the 103 cart (with paperclip) and drop it into the 401 inhaler, and puff away without issue.
I had asked because my 103 died and I had a ton of 103 carts, but no 401 carts... now I can use the 103 carts on the 401
I know it's better to just
buy 401 carts, but I'm having a hard time finding them... liquid is the other alternative.
Either way... 103 carts can be used on the 401 if you don't mind messing around a bit.