"ll" (double-l) ligature with stroke in the Book of St Albans

In a 1901 facsimile of the Book of St Albans (1486), I observe that the vast majority (perhaps all) instances of "ll" in running text are written with a little stroke across them:

Page 59:

nott the fowle. Oꝛ ellis ſhe will flie a littyll way afteꝛ and

Page 91:

How ẏe ſhall ſaẏ a gret hert (st) not a fair (st) oder
¶ A grete heꝛt when ye hym ſe ſo ſhall ye hym call
Bot neueꝛmoꝛe a fayre heꝛt foꝛ no thyng that may be fall

Notice that the "boldface" heading text has "shall" without stroke, and we even have "ellis" ("else") without stroke in the running text of the first snippet, but then we have "will, littyll, shall, call, befall" in running text. This seems to be the most common form of double-l in the Book of St Albans.

What's the deal with this cross-stroke? and where can I read more about it?

(Incidentally, there's another mystery to me here: that mark in the heading that I've transcribed as (st). I can't think of what it could be, unless it's a way of writing what a modern reader would call "parentheses" — "a great hart (not a fair)"; in which case the remaining mystery is that word "oder"(?) at the end of the line. Any information on these mysteries is also welcome.)

They are most likely flourishes with no meaning in English, although in Latin they would indicate an abbreviation, such as final "llus" abbreviated to "ƚƚ" or similar. In this text a scribe familiar with Latin has used similar forms in English, possibly for aesthetic reasons, although we cannot be sure of the reason. Latin words never end in "ll" so the scribe may have been more familiar with "ƚƚ" as a word ending. Variations between sections of the document probably indicate that multiple scribes were involved in its copying.

M Parkes's English cursive book hands, 1250-1500 (1979) discusses "additional strokes which in Latin text would indicate an abbreviation, but which may or may not do so in English": these are principally cross-bars over "l" and "h".

It seems to be something that's consistent for all cases of double-l in some scribes' writing but not in others. According to Middle English Grammar Corpus Manual, Merja Stenroos and Martti Mäkinen, February 2011

For example, hands that mark final <ll> with a cross bar generally do so completely consistently; this is borne out both by electronic searches of transcribed texts where the cross bar has been marked and by observation of the usage during the transcription and/or proofreading of at least fifty texts. The variation here tends to be between final single <l> and cross-barred <ll>.

Documents could be written by multiple scribes with different styles. In the linked document, other variations in letter forms indicate multiple scribes were involved.

For more information, I suggest you track down a copy of Parkes, which I was unable to consult.