Michael -
I'm not aware of any "inside-counterweight" locomotives. There are, of course, "inside-frame" and "outside-frame" locomotives, but this refers to where the drivers are in relation to the frame.
And since the drivers were always the same distance apart, more or less
" />, it's the frame that changed position. This is due to the locomotive boilers and fireboxes getting larger, which required the additional stability offered by widening the frame. As rods got heavier and driver diameter got bigger, counterweights had to grow accordingly, to the point where the designers simply couldn't fit enough weight between the driver spokes any more. So it was put outside the frame.
JAC