Turns out to be a SG Jr...
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What's up with the holes in the rear of the body???

You mean the lower bout?
Most likely the guitar originally had the screw-in Maestro vibrola ( not the more common long tailpiece type). The bridge would have been just a simple compensated bridge. The strings just slide over the top of it.
Here’s a pic of what I mean:
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Ok...
That makes sense to me. The owners want to replace what is missing...
The other challenge is finding the correct tuners. Because it has Schallers, I'm willing to bet the holes were drilled to 10mm and need to get reducing bushings. Or, it will need to be plugged and redrilled. Bushings are the better option.You’ll have two main challenges.
First, it may be expensive finding the correct Maestro vibrola. Do they want an original style or will an aftermarket reproduction be alright?
Second, you’ll need to get the right kind of compensated bridge. The one in the pic I posted is made for a wound third string, which was more common at the time. If you intend to use a plain third string, you’ll need to look for a bridge that has more of the Z-style (lightning bolt) configuration.
Assuming I install a vintage vibrola and bridge, I'm assuming $3,000 to $4,000 top dollar???

These are the style tuners you'll need. Juniors may have had an open back that year. My '68 Junior did. These are off my '62 Special. The shafts are 1/4" diameter, and I believe the bushing O.D is between 5/16" and 11/32" (.312 - .343"). I believe the one on the left (wound side) is a replacement from the fifties, as it doesn't say "Gibson Deluxe."
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DAMN YOU now I have white SG JR Gas!!!!!
I have to confess, I’m not the one to ask about pricing the guitar. I don’t really have a good insight into that.
My apologies.
you can make a CHEAP GIBSON SG.
You'd probably have to wait on getting that much for it in it's current state, or upgraded. There are many on Reverb and Craigslist in much finer original state for that much or less. Some have been there for ages. Fortunately for you and your customer, you live in an area with a built in high cost of everything and might get more. Unplayed, and not knowing what else is original, I'd offer $2K tops.Assuming I install a vintage vibrola and bridge, I'm assuming $3,000 to $4,000 top dollar???
Production numbers for SGs are higher than any other Gibson model. That's why they've managed to stay in production since 1961. For a workhorse guitar, they do their job. Just like with anything else, they may not be everyone's cup of tea.My point being, these guitars are worth very little from research and I'm cautioning the owners against investing too much money. This isn't a $90,000 1959 Les Paul.