I few years ago I made a Frankenstein Swift Saratoga fitted with Bak4 prisms from an Audubon model.
The difference in that the edges of the field were brighter than on a stock model, other than that little changed.
See it here....It's still alive...
i think you will find the Rangemaster has coatings on all the lenses in the ocular.
Mine does indeed appear to have low index prisms, however that is not necessarily a bad thing. Just as a digital camera with a high count pixel sensor sounds good, there are disadvantages.
Bk7 prisms are less...
The Rangemaster is a flagship model of the Bushnell, I have the same model but needing attention to an eye lens.
They are despite this fault sharp and wide but not as bright as a modern multicoated binocular.