That doesn't work in FE4. Remeber in FE4 an item is always that item. For instance playable item 2 is always an Iron Sword. If you save with it a silver sword it'll still be an iron sword without codes becuase it is still just playable item 2.
What the OP wants is the items he missed. He needs to get them to appear somehow. Not turn existing items into them.
I did come up with a code sometime ago (3 or so years...) to make any of the legit in-game weapons available in the shop (character storage and character inventory get a bit confusing but shop ware easy enough). I can't remember the offsets sadly but for example:
7e3d8b** (Alec Iron Sword Weapon Modifier)
7e3d8c** (Alec Iron Sword Usage Modifier)
7e3d90** (Alec Iron Sword Kills Modifier)
It is between 7e3d8d and 7e3d8f (so thats 3 possible locations to try). I remember ** being 04 for shop but I may be mistaken...
Once you've found it its easy to recreate for any of the weapons.
Edit: In the above example it is 7e3d8d04 is the code you would need.
The patten is just +1 to the "Usage Modifier" offset and then make ** 04