Someone gave me this years ago and they had no Info. I've look very close and no markings anywhere.
Footprint Approx 3x4 ft. (Is currently buried in garage again to get more accurate size.) Seems Vacuform Polypropylene. Someone added "Grass" to unused spaces and "Water" in the lake but most areas are original.
Front. Track shown was just playing trying to figure out track plans. Some areas in Tan and "small valley" on small mountain is for expansion out of this space. Believe some access holes under the mountains are expansion ports too.

Back. The OE Bridge was longer then 1 strait track and thus standard bridge kits won't span the gap.
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Close up with Standard Bridge base on 1 track section.
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Bottom. 1 OE Paper based panel (
Can't remember Trademark of that Masonite(R) panel was sold under in 60-70's.) on bottom to support track going thru smaller mountain.
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Note: The product Might not be made as HO but OO or related as might be imported. UK OO track is same but trains are not exactly = to HO with OO trains are slightly bigger then HO.