but it seems Puma is the only one that has 'phase armor' working as Firesparks called it. Stuart has same armor as m8 but you don't see missed shells going through stuart like you see on puma. (nor on m8 t17 stag ac for that matter)
M8 t17 stag acs - when shell misses , but hits the hull , damage is calculated
puma - shell misses, but goes through the hull without damage like puma would be a ghost
m8, t17, stag, acs, and puma all have ghost armor against tank shell.
the stuart and hotchkiss do not have phase armor.