Wedgie,
My understanding is that this is a small tank that was used primarily with the mark 20H model. The ears were that way from the factory, I believe, to trim weight. The air hole may have been plugged to convert it for use with a single line and a fuel pump, although I believe the mark 20H did not come with a fuel pump.
Later when Mercury put fuel pumps on most of their motors, those tank tops continued to be cast with "air" above the spot where a hole would have been to accommodate the air line. I should have a photo and I will try to post it here.
There are many Mercury experts here who will chime in and let us know for sure.
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