Thank you all for your help!
I was able to contact my supplier... and what she told me was essentially what Feline Fanatic just posted. In this case (Magaglide pre-wound) the core for the L and M class bobbins are the same - the M class just holds more thread than the L does.
As another note - I was more than sufficiently compensated for the substitution so everything is good! Just the same, i will do a practice sandwich before starting my next project (which is my practice anyway). I have been assured from the supplier that i can return the L’s for exchange and replacement at no cost to me should i decide not to use them. It will just take another month to get that particular color in the M since they are backordered.
To be clear though... no - do not substitute actual original equipment metal bobbins (L for M - or M for L). That i would definitely not do! As Feline Fanatic reported, the Magnaglide pre-wounds are a different situation. I will update you all once i actually use the L, but it wont be right away because my work-life seems to be getting in the way of my quilting-life! 😊
Thank you all again!