yep positive, round only on 90 spec, square is pre-90.
As said '90 spec' actually came around in 1989, but VW would have had to work fast to get an 1989 90 spec made and shipped to the UK in time to be sold before 1990. You also got a fair few pre-90s being registed on a G plate since they didnt sell quickly enoguh, so the line can be quite blured!