Tractor Sizing What backhoe for ford 1620?

[B Rive]

I am looking to add a backhoe to my tractor and have been doing a little research but still a little confused about size attachment for my tractor. With the New Holland attachments there is the number,ex. 756,757,758, and there is the letter, A, B and C, after that. What exactly do the numbers refer to
And what do letters refer to?
Also is it possible to make any brand attachment fit?