On Tuesday, March 1, 2016 at 3:43:37 PM UTC-5, dpb wrote:
On 03/01/2016 10:50 AM, wrote:
...

I'm an engineer; I can look at numbers on a piece of paper and
sort out how things will (or won't!) work without having to
"try it and see!"

The easiest way to determine a masonry R/O is to drill a small hole in
the jamb near the stop and probe with a wire. You can easily fill that
hole and make it disappear if you decide not to replace the jambs.
You might find there is quite a bit of bucking in that opening.

+42

Nobody said rip the whole thing out; it's unlikely to the max the RO
wasn't plenty for a "standard" door and guessing from the exterior fill
isn't much for uncovering what is actually there.

Sometimes even we engineers need to do lab work, not count of design
drawings...

--

I'd just pull the trim molding off to see the RO. Assuming he's replacing
the door, it has to come off anyhow. But then again, the scope of the
actual project hasn't been stated. It's almost certainly a RO where you
can find a replacement door to fit, there are probably a lot of choices.