I stand corrected....on a number of accounts it seems! I will update my notes.
So it seems that Princess Margaret's first tiara wearing was in September 1948 when she worn Queen Mary's Lozenge Bandeau. (Thanks Marianne and LDMJ)
Queen Juliana's Inauguration was just after Princess Margaret's 18th Birthday (Aug 21) so perhaps I was wrong too about my supposition that the daughters were treated equally.
I did not recall Roberts mentioning that the Scoll/Halo was QEII's 18th birthday present (Thank you jinja and Beth for your insight) but the "early" tiara worn by Margaret seems to be the Scroll/Halo which, according to Roberts, belonged to QEII. (Thanks Nellie for the 1951 Paris photos)
Therefore, it stands that perhaps Margaret did not have a tiara of her own before the Poltimore???