I was talking about the unique 1926ME paired with the 1927 reverse, what were you referring to? That one CANNOT be called a mule as it obvious that it was the intended design as we see the following year with the 1927. No-one (at present) knows when the unique variant was struck, i.e. whether before, during or after ME dies were used for the end of the 1926 penny run, but by any definition of the term 'mule', it isn't.
I was theorising, that's true, but I felt it was the best explanation (so far) for the several questions posed by the mere existence of the 1926 penny. As for experimental dies, are there any obverse dies that weren't in fact used for currency runs? Even the short lived 'recessed ear' of 1915 and 1916 was used on several million pennies (though it would be fascinating to know why that was abandoned, as that particular experiment resulted in fully struck up Britannia reverses which the normal obverse did not). The only experimental die I know of for sure, is the 1922 so-called "reverse of 1927" which only exists for a few specimens and was never used again.
Yes, "pattern" seems more appropriate than "mule". We'll have to leave the normal 1926ME as a "grey area" and agree to differ! Though I will say that 1902 HT and LT is less of a controversy as that design changed partway through the run, and clearly 1902 LT was the first design as per 1895-1901, which then got changed to HT as per 1902-1926 for reasons we may never know, i.e. neither 1902 is a mule. My main case for the 1926ME is that (theoretically) the ME dies were grabbed to finish the run but they had enough reverses to do the job. In other words, the pairing was "deliberate" but not "intended" by which I mean that if normal circumstances prevailed that particular pairing would not have been used.