There are indeed differences between the two boxes. I have both the v3 500 combo and v3 210 ext cab...
The ext cab is slightly heavier than the combo due to the denser wood used (China vs Indonesia wood).
The combo has damping material only on the bottom half of the back wall. The ext cab has damping on the full back wall and both sides.
Internal volume is possibly subtly different - the combo has the cutaway for the amp, the ext cab has the recessed handles.
The combo has 128mm length port tubes, the ext cab has 106mm length port tubes.
Do the knuckle-rap test and you can hear a BIG difference between the two boxes - the combo has a bright / zingy / echoey response, the ext cab has a low dull thud response.
Same speakers and tweeters but the boxes sound different too, the combo has more ‘air and presence’, possibly due to the above differences. The ext cab feels and sounds more solid and tighter overall, which I prefer.