1, The models with Reg. Tm are the 1950 case models. The Reg. TM is the English equivalent of Brev. Ital. which was engraved on some of the original vintage watches.
2. The op XI is the same base movement as the opII with the addition of a swans neck regulator and different finishing.
3. Plenty of amplitude is probably a way of
saying the movement is in good condition and the mainspring is strong and the movement is clean and well adjusted.
4. The swan's neck regulator allows the movement to be adjusted with precision more easily. It doesn't necessarily mean the movement will keep better time, it just allows for a fine tuned method of adjustment.
(these are my layman's responses others may be able to chime in with more accurate info)
"But sometimes when you bring the thunder, you get lost in the storm." K.P.