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Kerning on a Monotype Keyboard is a "unit subtraction function" which tells the mould to open up less when casting type. This causes the type to be under cut, or rather, under cast. Kerning metrics are completely an "operator" decision, nothing "factory predetermined", nothing included in "font information". I am not speaking of the italic lowercase y, j, or f etc. which "is" part of font metrics. In fact, genuine keyboard kerning procedure were a discovery, not an intended engineering feature. Although later included in the manuals. It is a myth that Monotype had specific hot metal kerning font metrics.
I might add, there was a secondary method of kerning which required manual insertion. That method is the same but takes less skill and more labour.
"SPACING" was "variable" unlike hand set types which has fixed units. For instance hand set type used quads, en quads, three to the em, four to the em five to the em, six then on to hair spaces, 1pt, 1/2 point, some shops such as ours cut papers for 1/4 point spacing. Monotypes were far finer.
Spacing goes between the words to justify lines of type independent of specific font metrics although part of operator considerations. If you are seeing, as you say certain "advance widths popping up" they are a digital error. If in hot metal, they are an operator error, no fault of the system.
Now I do not wish to give you a headache but there is a thing called "fixed" and "variable" spacing. Sometimes you would use one, sometime you would use the other and at others, good operators may choose to use both. This system should be included in modern day page layout procedures. If you call me I will give you an understanding of its advantages.
Lanston never had a photocomp system. Monotype England did. The keyboards were the same, the output device had negatives replacing the brass matrices. The system otherwise was based on the hot metal system.
If you are seeing something as you say you do it may be the following. As I remember Monotype England sold digital laser printers and systems to the newspaper industry. What you may be seeing is the evils of "early digital technology". And John and others are blaming it on "hot metal technology".
I wish to avoid falsehoods getting foisted upon the unknowingly by pretenders.
Furthermore I am not stupid, or "commercially evil" as one member has indicated. I know you and Adobe often suffer from similar abuse.
But before I go I must thank you for the opportunity to clarify matters about the Monotype System.