I have an English Parker 51 Vac in the same color that I recently sold to a friend,and the hood (the part that looks to be see-through on yours)on mine has a similar transparency. I suspect that the batch of lucite(plastic)that yours came from might have been a little thin in color. As long as there are no problems with the hood,it should be fine.
John
Been known to happen... just a vagary of the molding on that particular batch...
I have two vacs (both from '46) and both of their hoods are semi transparent. I have been told this is not too uncommon phenomenon.