Hello Peter & Mike,
The box lost about 6" in the transfer to Caerphilly. I knew this, as the lead off timber was at ground level, instead of being much higher. When I started at Caerphilly, they were about to scrap the box, as no-one was interested in seeing it restored. The locking engineer at Newport still had spares! The original down distant at Rhiwderin lasted well into the eighties, still with its somersault apparatus. I understand the 11 lever frame is still with the surviving Caerphilly guys, now located at the Gwili Railway. The reason that the box is much altered is because a lot of it didn't make it out of Caerphilly; At least, I seem to remember that most of the basement stayed after the Teifi had removed the top of the box.
Non standard slates on the roof.... I seem to remember them being narrower than the standard sizes.....
Regards,
Ian