Hello Steve!
Fun isn't it!

From the photo of the backside of the solder sample it still appears to me that this is either ImAg or ImSn.  When you or the vendor did a solder test on the front of the board, this backside simply reflowed with minor pooling in the center of the pad.  I don't see any copper bleeding through nor can I see any oxidization (The lighting makes that somewhat difficult in a photo). I also don't see non-wetting and clearly not de-wetting.

If it was HASL, I would expect to see more solder, but even that is arguable.  Unless HASL were very flat I would expect to see minor pooling along the same side of each pad.  I don't see that.  Whatever it is it looks like it was reflowed.

Glen