Re abc John, it says any line beginning with a letter in the range 'A-Z' or 'a-z' and immediately followed by a colon (:) is to be interpreted as a field. I think a leading space would break that one as it would "extended information fields", ie. lines starting with %%.

In practice, some abc software will handle it (some producing warnings and others silently) and others will fail. In other words, it's "unpredictable"...