Thanks a lot for the precisions !!
It is well worth investigating as there virtually no information available.
I was in contact lately with Michael Holzapfel who took over his father's business with his wife. I was asking about some historical background.
It seems to me that the Holzapfel Little Willy has been produced in fewer examples than the L-S LOC traction Engine.
Your picture showing it at the side of your Minnie is wonderful.
I noticed this morning that Holzapfel rejuvenated their website which has a way more modern look now.