Difference between Quad ESL63 Pro and Quad USA mon

What are the difference between ESL63 Pro and USA monitor and normal ESL 63? Some said difference in steel gill ( pro ) and aluminium (USA monitor and normal ESL 63); Other said both pro and USA monitor use still grill while normal 63 use aluminium grill. Please advise.
"Crosby Mod" type of grill (steel), pro audio speaker terminals and side handles mainly.
The ESL63 Pro has Neutrik speaker connectors in addition to the 5-way binding posts. An AC power cord locking collar, side handles on the frame, rubber boots over the top and bottom of the grille cloth, and the top wood panel bolts to the frame.

The protective screen, grille cloth, frame, electrostatic elements, electronics, etc. are identical between the Pro and Monitor versions. The two speakers are sonically identical.

The Pro version was developed for professional applications so the need for Neutrik connectors and power cord lock. They get moved a lot; Quad added side handles. And since they need to be moved around a lot, the top cap was bolted on to keep it from falling off.