Hi there,
a nice gren with the early small ringpull which is not so commonly found these days and in very good nick too. All the other parts look good too. not all the base plugs were marked so its not that unusual to find one like yours although the type is again not so common.
The shield on the front wont give you a clue to the maker ime afraid, its only a factory or foundry mark and quite a lot had this. Also, a nice example of a flat lever. Certainly go ahead and complete the gren by attaching lever to striker with the spring under tension, wont hurt it at all, after all, thats what it was designed to do. Just remember not to pull the pin with the base plug attached. If this still worries you, fit the striker without the spring, the gren will still look the part.
Nice gren, i would be proud to own it.
Andy