A lot of the box brownies are 620 rather than 120.

Not a major problem if it is - you usually just need to file down the rims of the 120 film spool, and it will work fine. use the 620 spool thats in there as the take-up, and make sure you don't loose it - 120's work less well for takeup, so you need to keep reusing the same 620 take up spool.