It is a common trick, and immediate promotion (within the same year) is extremely unlikely to the next grade no matter how much they promise you. Unless it's the Managing Partner who makes that promise and you get it in writing. If I was interviewing you, from what you've described I'd place you at SC/M. I would need to be pretty sure that you could hit the ground running as a Manager and your experiences to date backed that up. Expectations of Managers within the Big 4 are extremely high