Vilbus of Guernicus has learned the Frosty Breath of the Spoken Lie spell as an apprentice and used it regularly.
During his last encounter with a possible oath-breaker, Vilbus ended up in Twilight, but managed to master it - he came out with the minor Virtue Potent Magic: Truth, which the troupe decides applies to Frosty Breath of the Spoken Lie. Exellent.
Vilbus now decides to invent a version of the Frosty Breath of the Spoken Lie that takes advantage of his new virtue - he needs a snake's tongue to cast it, but it has better penetration.
The question is: Does Vilbus Spell Mastery with Frosty Breath of the Spoken Lie carry over? Is this really the same spell, or merely a similar spell?
Technically, it is a different spell. Though Similar. The downside is you have to relearn a new Mastery ability, but the upside is that you have a range of potential Potent Truth spells. I myself would bend the rules and let the new spell be considered same, but you have to discuss that with your troupe. This exception would only apply to that spell (it caused the Twilight, yes?). Future spells would not cross over.
I think that HOH:TL (or was it the mysteries?) states that piotent spells are similar but not the same spell. I would apply a "similar spell known" bonus to his lab total, but mastery points would certainly not carry over. They are 2 different spells.
This scenario appears to fall through the cracks a bit. There is a key difference between the spells, that of the casting item used in the Potent spell. They aren't identical spells, they are nearly identical even though the base, range, target and duration are the same, the addition of the casting item is enough to change it. After all, if they weren't different spells, you could just use the casting item with the old spell.
My feeling, and this is just a feeling, is that Adaptive Casting might be leveraged here. Adaptive casting is supposed to be used only for general spells, but I think this use is a good fit. Adaptive Casting mastery is part of the Cult of Mercury and may be restricted, which would require a story to pursue. I think (totally speculating) adaptive casting was limited to general spells, because it was primarily intended to prevent combat monkeys from putting it on Ball of Abysmal flame and unlock all of their master in Pilum of Fire. It's clearly not being used for that purpose here.
Adaptive casting isn't without it's problems, though... Is the mastery score of the other spell 0 or 1? Technically adaptive casting is put on one spell, and it opens up the mastery abilities to the other general spells of the same type...
I'm very familiar with it, and I must admit I disagree that it would apply. I could however see it as inspiration for a new mastery ability with... largely the desired effect.
But it seems to be the opinion of the board that these would be different spells, so something would have to be done to have a mastery score cross over.
Which is why I said adaptive casting could be leveraged. I'd be reluctant for there to be too much fiddling, otherwise one might see mastery scores for Pilum of Fire transferring to Ball of Abysmal flame. That would be...bad.
Maybe if you can specify that spells of the same level, sharing the same base and RDT parameters may be treated as general for this mastery ability?