I grew up Baptist before entering the Church, but, yes, they believe it was grape juice. They understand this bit of Scripture as grape juice:
Mt 26:29
“I tell you, I will not drink from this fruit of the vine from now on until that day when I drink it new with you in my Father’s kingdom.”
Mk 14:25
“Truly I tell you, I will not drink again from the fruit of the vine until that day when I drink it new in the kingdom of God.”
Lk 22:18
“For I tell you I will not drink again from the fruit of the vine until the kingdom of God comes.”
Having attended Catholic schools in my youth & having had many Catholic relatives, there were many opportunities to attend mass, so I knew of our different understandings of these verses, but, yeah…They used (& still do use) grape juice as did all of the Nondenominational & Fundamentalist churches I’d attended in the past.