It was explained that the 350 and the 520 are interchangable only one is older, it was also explained that sometimes you will not be able to tell if you get a used part for a 520 you may be getting a 350 part which may be older and if so the metal may fatigue sooner than a more recent 520 part.
also the newer s20 is the burner plate smaller or larger than the 520.
I ask these questions because in the event I rebuild my 520, the newer the assembly the more efficent the heat will be. (slight changes in the draft holes and plates could make a big or worse difference).
also there is a british coal mechanisim that is supposed to be traditionaly leaner to run ie less coal and hotter heat without distoring the burnere mechanisim which I do not fully understand, all I know is that with any combustable a minimun amout of oxigen is required to burn and the more 02 the hotter the heat however with newer metals being introduced it may be feasible to upgrade the burner and run it hotter with more 02 and a leaner coal feed and be more efficent that what was done in the past with cast parts of older forged metals (there are newer atominacil combinations available everyday, even adding ciramics to metals) and being less brittle and more durable than we might find on the shelves today. thank you and sorry for being such a dull person.