Reinforcing the floor.

[Lumatic]

I'm talking 73 Streamline here. 5/8" ply subflooring. There is more floor flex than I would like between a couple of the plywood sheet joints. Feels like a lack of support underneath, not rot.

I have the floor stripped down to the subfloor.

I am thinking about cutting a small hole in the floor to see what is going on and then scabbing in a support board that I would glue and screw to provide the joint with more support.

Anybody have any better ideas?

[DryFly]

I've done this several times on floors in houses, it's probably the easiest thing to do to check below the floor. You might want to cut 2 holes adjacent to each other rather than one larger hole. That way you can shine a flashlight in one while looking into the other, then switch. Also if both holes are in the same bay of the subfloor space, one piece of wood can be used to act as a brace to patch both holes. I would recommend that the support piece be 3/4" plywood as screws won't split it. I've never glued the support piece in place as it makes it very hard to re-access the area should you need to in the future - may not be an issue for you though.
My bet is that you've already determined that the floor and frame is in good shape.

[Lumatic]

I didn't cut a hole in the floor because I found a Streamline exploded view in which it looked like there was likely to be a frame cross brace under the subfloor sheet seams.

I drilled a bunch of countersunk holes and most of them caught metal. Since the seam was still a bit depressed I put a fiberglass patch over the seam.

Next to sand it flat after it cures and finish the vinyl floor.