OK, here's the picks. From the description I thought it might be a jack smelt, but it in no way looked smelty. Looked just like a sardine, except for being a little chunkier and the coloring.
Top is mystery, bottom sardine.
Order inverted
Also, you can't see it in the pic, but the anal fins were much more widely spaced on the mystery fish. I had two of them and the one I dissected was full of roe. My off the cuff guess is some variation or related sardine species, but there was enough difference in the overall anatomy that I'd believe it if it was a diferrent species altogether.