Those ones are only for trapped piping or manifolds. The one you want for the boiler is this.
Find many great new & used options and get the best deals for Bell & Gossett 112114 EASB - 3/4'' Sweat JR Air Seperator at the best online prices at eBay! Free shipping for many products!www.ebay.ca
View attachment 32389
Inside the brass air seperator is a stainless screen that pulls micro air bubbles from the boiler loop. After a few days all the air is gone from the water. The one you used will not do that. It's simply for trapped air. Oxygen causes corrosion. If the one you used is for releasing trapped air in a section of the heating system it's fine. Those are exactly the ones I use.
The photo is a boiler I rebuilt last week.
On something like that, would I be able to block off the two side inlets and put a 1/8 on the bottom? My setup comes directly off of the top of the boiler as opposed to being inline with the pipes. (Pretty sure the one I posted is what this shipped with back in the '60s / '70s )