Nothing else is custom other than the color and lack of pick guard. There’s also a Fishman Pick-up in it. See here:
Hey Brandon, welcome aboard!
Maybe just casual usage of the term on your part, but I wouldn't call that "custom" as the 2 details you mention could have been had simply by ordering the guitar that way.
In fact the finish color just looks like their regular "Mahogany" stain which got progressively darker through the '70's as seen on D25Ms.
Now if for example the inlays weren't standard on the F48, or it had D55-style neck,
those could be considered a true "special order" or "custom". Those guitars were usually but not always ID'd as "spec" after the model number on the label.
"Custom" was usually reserved for guitars built in-house without a special order with some definite off-spec feature(s) compared to the standard model.
A pickguard delete could be special ordered if they didn't actually build it that way (and we've seen other examples) but again, I don't think that makes it a true "special" any more than the dual-guard guitars which were popular at the time and which could also be ordered if desired.
It looks like the label's missing, it would have been handy to see how it was actually labeled. My guess would be "F48M". Nothing special.
I agree with the folks who say "Condition is everything". Not only that, but I think there's gotta be a reason that model never really lasted.
I'm thinking a 'hog bodied true jumbo just doesn't grab a lot of people, or else somebody else had a better offering in that niche. So even if it were a true custom I don't think it's all that desirable which also holds back value, except to somebody who does love a hog bodied jumbo.
I'd look for sold prices and deduct for condition.
Hope that helps!