He is a Vietnamese clam. His incurrent syphon is open like that all the time like a Crocea. IMO, his eye spots pattern is somewhat like a Crocea and the shape of the mantle is also more like that of a Crocea. His shell is not as elongate as other Vietnamese Maxima. His color pattern is not at all like other Vietnamese Maxima. IMO, it is very possible that we have a hybrid of Crocea and Maxima here.
I have not look at his byssal opening yet other than when I put him into the tank to check for parasitic snails. I was putting a large number of clams into my tank so I actually don't remember what his openings is like. I have to pull him and take a picture of it later.
Minh