Hi Harry, welcome and Happy Christmas! Do not worry about your English, it is fine and better than some English speakers.....
Clive Elliott will tell you all you could ever want to know about Tabby and he has already written comprehensive articles on here, a search should find them.
Richard is correct, it is LD for leyland, the BC6**** means it is likely to be Comet, Centaur and Cromwell generally being BC3**** and BC4****. I have a few of those, Chris Wilkinson had several......
There is a makers number, such as C/84 or L/124 and the census number such as T189557, neither of which are stamped into the hull. The plate Rick mentioned, being cast in brass, is often missing!
The tube is sleeved because the original 'deactivation' when it left the army simply consisted of cutting the tube in half. It had been rejoined when put on display but it was not quite straight! We re-did it and used a tubular sleeve to get an accurate alignment.
The deactivations consists of welding a plug in the chamber at least 2" thick, and welding bars across the rear opening of the breech ring. If it had a block, the bars would not be necessary but the block would need to have a cross cut into the front face and the firing pin hole welded up.