How to remove air vent slug from new OCS Cast Iron Radiator

[Jakedeg]

I just bought three new OCS Cast Iron Radiators to replace some broken ones in my house.
The way they came new from the shop, there is a little plug filling the 1/8" threaded tap where I need to put in the air vent.
I can't figure out how to remove this plug. Is is threaded? What can I grab on to? There's only a small curved ridge exposed.
There must be some trick that I'm missing.

 

[Jakedeg]

Ah! Solved my own problem. Got it out pretty easily with a 3/16" Allen wrench.