I was trying to rebuild the LEGO Harry Potter Hogwarts Castle from 2001 and I’ve noticed that at some point they changed the 1x2’s model. Which design is older?
I remember seeing some new bricks reusing (probably updated) older variation of the mould. So this kind of information isn't always accurate.
Here what Rebrickable tells about variations of Brick 1 x 2:
There are three known mold versions of this part:
3065 Brick 1 x 2 without Bottom Tube This is the original version, introduced in 1955. The inside has no bottom tube. While it was replaced by a tubed version in 1959, it was kept in production for transparent colors.
3004 Brick 1 x 2 with Open Bottom Tube This seems to be the modern version. The bottom tube is open.
3004 Brick 1 x 2 with Solid Bottom Tube This seems to be an older version. The bottom tube is solid.
And since Rebrickable's idea is to keep accurate inventories for each set they have following disclaimer for this brick:
The mold comparison image shows all three versions. As long as we are unsure when the mold change of 3004 happened, we cannot create different parts for these molds.