What is the third lug on the E receiver?

[Bang]

The E-model receiver has an unused threaded lug between the two lugs that are in use. I understand the E receiver is used for 4 rifle models. I assume the lug not used in a mark II .22LR FV SR is used on another model? It looks like I could drill the bottom plate and stock and add a third bolt. Might be a way to improve receiver stabilization.

[Bang]

Got the answer....it is there for differing trigger guard setups.