Reading it again, I'm still not sure I understand.
I think what you want is to mark a place on the map, where the player needs to obtain an item. Not sure how that works, but look at the Sapienza mission for an example. You might need to add a modded brick to the level, then you can set your own id in the brick and use that in the contract.
I do not think there is a way to do that by only editing the contract json, as weapons are not unique.
The modder tool of the framework can convert TEMP/TBLU to entity.json, download it on this page. You can then create a new entity.json from scratch based on what you learned from reading entity.json of sapienza files and a few bricks to understand how bricks are set up. Although the format makes it easier than it was before, it will be much harder than editing a contract and I cannot help you with that step!
Once you have your brick as entity.json file, add it to a raw folder with a TEMP.framework.json file to set its name to use in the contract. Look at the unrestricted loadouts content pack for an example. You will need to replace entitytemplate with entitytype as that pack uses scenes instead of bricks, but if you got this far you probably figured that out by yourself. ;)
