There are alternative glyphs for Devanagari numerals ५ (5), ८ (8) and ९ (9)[1], that look like cursive "y", ८ with upper horizontal bar, and mirrored "3", respectively.[2] These are sometimes called Hindi[3] or Calcutta[4] forms, while the "default"[1] ones called Bombay forms; sometimes all the variants use the same glyph for "9", and the "mirrored 3" form is considered Nepali.[4]
The alternative glyphs are not resolved into different Unicode characters. So can they be considered an analogy to "a" or "g" typographic variants? Or are some variants used and understood more preferably or exclusively in some (which) regions or contexts? Is there any good "open access" source explaining this phenomenon?
[1] note: I assume the (default) glyphs in this text are those presented in common Unicode fonts, like Google Noto, i.e. "Bombay" forms.
[2] Scriptsource: Alternate digits in Devanagari
[3] H. M. Lambert: Introduction To The Devanagari script, p.32 (PDF)
[4] Devanagari for TeX, version 2.17 (manual), p.21 (PDF)