Swollen calyx for sure.
I also like to collect these as it gives me a early taste without picking at the buds.
This swelling of the calyxes is what occurs to your buds when they ripen and is why buds pack on weight in the last weeks.
Sometimes these swollen calyxes are called false seed pods, which is abit of a misnomer, a swollen calyx is just a sign of maturity of that particular calyx.
In some strains the calyxes swell more than other strains and gives them that bunch of grapes look.
Peace
Growerz