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[45PSS]

Picture 1- standard double sided film holder top, film pack adapter bottom.
Pack film was discontinued in 1970-1971. Use it as a decoration or paper weight.
Picture 2- standard 2 sided film holders, they accept sheet film.

Picture 7- going from right to left, board with two lens mounted on it attached to an adapter plate which is attached to a Packard shutter which is attached to the front standard like a lens board would be.
Does the Packard shutter/stereo lens assembly detach from the front standard?

Picture 4- there appears to be a divider and some sort of reducing curtain at the film plane. This is a definite modification that will need to be removed to use the camera as a standard 4x5.

The most common lens supplied with the camera were the 135mm f4.7 Optar and the 127mm f4.7 Ektar. Both are moderate wide for the format.

B&W film in 4x5 format is around $1 to $2.50 a sheet. Most develop their own.
Color print film is available in 4x5 sheets for $3 to $5 a sheet.
Color slide film is $7 to $10 a sheet.

Processing costs depends on the lab but possibly another $3 to $5 a sheet or more.

120 roll film backs are readily available as is 120 film and processing at a lower cost.

[rjb1]

The Signal Corps had a number of photographic duties, one of which was to take photos of terrain to be attacked, for planning purposes. A stereo usage would be useful for that.
Another possible use for your camera (and I tend to think this is the most likely) is to take two identical small photos simultaneously for Photo ID purposes.
The soldier keeps one and the Army keeps the other for records use.
A small photo (~ 2x2) was part of the standard-issue ID card for Army personnel.