I don't know how that outcome of the item becoming out as leather protection is obtained, so I can't really say....
But this:
[ARMOUR_MATERIAL:iron]
is completely valid entry and there should be now issues with it.
However, as the recipe creates "iron spectacle helm" item you don't really have to even specify ARMOUR_MATERIAL or ARMOUR_COVERAGE or WEIGHT.
Especially as these don't seem to have changes to vanilla item.
The price has been reduced significantly here for some reason. :/
.... the player characters seem to be super fast and budget friendly iron crafters compared to the village masters