Many companies over the years have produced a knife which they called a "Texas Jack" according to catalog images in several guidebooks I own.
Some of these knives were based upon patterns as diverse as sowbelly, double-end premium jack, premium jack and stockman frames to name what comes immediately to mind.
So right off you can see that there is quite a bit of latitude taken by the manufacturers. Generally the "Texas" in "Texas Jack" implied that the knife was a
larger knife (relatively speaking).
Now, according to Levine's Guide, a "Texas Jack" is essentially equivalent to a "Moose" which is a large (usually 3.75" or above) knife based upon the premium stockman frame with a wide clip master and
either an equally long spear or spey blade on the other end.
Steve Dick, in his book on pocketknives narrows the "Texas Jack" moniker to only those with those sporting the spear blade while those with a spey are referred to as a "Moose".
Hope this helps somewhat in terms of definitions...(though it certainly doesn't cut down on the license taken by the manufacturers).
To elaborate with a few illustrations, each of the knives below has been labeled a "Texas Jack" in guidebooks or catalogs:
The original "John Primble" version of this Moore Maker was called a "Texas Jack":
This original "Winchester" was referred to as a "Texas Jack":
This "47" Pattern "Case" was sometimes referred to as a "Texas Jack" in guidebooks:
And finally, this "75" pattern "Case Classic" can be referred to as either a "Moose" or "Texas Jack" (according to Levine's Guide):
So there you have it. Murky as ever.