Unbelievably I missed seeing these posts. I think you've made a very valid point. The two 6's are indeed different, and there is absolutely no reason why they'd have two virtually identical experimental dies each with a different 6 stamped on, just to strike a few coins.
The other suspicion - though it might sound rather trivial and inconclusive - is the colour difference; 1926 pennies are usually darker than 1927 onwards which usually have a paler more orangey hue. But as I say, that's not really conclusive unless someone has a record of an alloy change made in 1927.