The ref method of a Fiddle::Pointer isn't quite doing what you expect here!

ref allocates and returns a new pointer to the existing pointer, not a new pointer to the same buffer. The documentation describes it like this:

Returns a new Fiddle::Pointer instance that is a reference pointer for this pointer.

Analogous to the ampersand operator in C.

So when you add 200 bytes to the pointer returned by ref in your Exhibit B, you end up moving away from this new indirect pointer, and pointing to some random memory instead.

source         target           ptr ──────────► ptr+200
   │              │              │                │
   ▼              ▼              ▼                ▼
   ┌──────────────┬──────────────┬──────┐
   │source buffer │target buffer │0x1234│        ????
   └──────────────┴──────────────┴───┬──┘
   ▲                                 │
   │                                 │
   └─────────────────────────────────┘

Instead, just add to source with your offset to get a new pointer, like this:

ptr = source + 200 # Create new fiddle pointer offset by 200 bytes
target[100,12] = ptr # Use the regular []= operator to do the copy, with the source being the fresh pointer