This is incorrect. If @GabrielOB said so there's been a misunderstanding. 3403 MOD means the style 3403 has been modified, in the case of your shoes there's been an addition of a medallion to the existing style. It does not mean it's on a modified 946 last.
There is actually no modified version of the 946 last. All our styles on the 946 last is made up in the same way, in standard F width (normal with).I ask you to direct all further questions in regards of this matter to my personal email address. Gabriel can provide it, I'm on vacation now and won't be able to respond until next week though.