I know about these holes in the firebox. The very first Jensens were built with open flame heating requiring the vent holes. At the first major toy show that Tom Sr. showed them, he received his first order for 50 engines contingent on them all being heated electrically. Rather than make new fireboxes, (remember, Tom Jensen Sr. wasted nothing), He simply added electric heating.
This engine appears in Jensen's earliest known advertising and the cut was used throughout the 1930's and into the 1940's even though all the engines sold were electrically heated.
Hope this helps,
Gil
Here are two of the early ads and both have that ultra rare Big Power Plant
with the vented firebox and slide valve engine.
Notice even the artists cuts in the ads ads have the electric cord.
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