Anyone know why there are different part revisions, even in same year?

[hagridshut]

I recently purchased a Stellar Recon Voyager on Bricklink.  The set is listed as having two of these components: Container, Box 2 x 2 x 2 Item No: 4345. 

However, the copy I received has only one 4345 box, and the other box is one of these: Container, Box 2 x 2 x 2 - Solid Studs Item No: 4345a.  The only difference is that the 4345 has hollow studs on top, while the 4345a has solid studs.  

For functionality within the set, the components are exactly the same.  As installed, the difference isn't visible, because a 2x2 magnet assembly is connected to the top of each box, concealing the studs beneath.  Many people might not have observed the discrepancy, but I notice these details, although in this case I don't care enough to chase down an extra 4345 to replace the 4345a.

Curiously, the Stellar Recon Voyager's smaller relation, the Particle Ionizer, released the same year (1990), uses the 4345a rather than the 4345. 

Does anyone have any insight into why LEGO Group might have used different revisions of the same part in the same year?  

[JopieK]

There sometimes might be reasons that they change / optimize the mold, I think that is the most important reason for such an updates.

[Lyichir]

Lego usually has more than one copy of each part mold, especially for more common parts. So when a minor revision is made to a part, it would be rather expensive to replace every mold for that part at once. Instead, new molds are often "phased in" gradually as each older mold wears out and needs replacing.

[JopieK]

@Lyichir, yups, thanks for the addition. We visited Kornmarken a few years ago and the explained that for the more difficult parts they keep those molds only in Billund (amongst others to protect their IP) and for more common parts they make the molds in Kornmarken and ship them also to Mexico / China / Czech Republic. For common bricks / plates they have multiple machines producing these parts in each factory so they indeed have a whole series of e.g. 2x4 brick molds. 

[hagridshut]

19 hours ago, Lyichir said:

Lego usually has more than one copy of each part mold, especially for more common parts. So when a minor revision is made to a part, it would be rather expensive to replace every mold for that part at once. Instead, new molds are often "phased in" gradually as each older mold wears out and needs replacing.

That makes sense to me.

I suppose it is possible that the 4345 molds were projected to wear out, so LEGO started making 4345a pieces for some sets.  This seems like such a trivial difference, but it would be fun to find out the true reason the component revision. 

[splatman]

A hollow stud requires less plastic than a solid one. It's just that, apparently, no one thought of that in the first place. When the idea did come about, it was implemented into every new mold made since. There were probably already loads of 4345 molds when the 4345a molds were put into service. Since the 2 designs are functionally the same (save for some additional possibilities afforded by the hollow studs), they are used interchangeably, resulting in the 2 variations appearing in the same set. Since the molds are expensive to make, they are still used until they completely wear out. Replacing a mold early would negate any savings afforded by the new design probably by several orders of magnitude.

A more everyday example: You find a car that gets better fuel economy than the one you already own, but buying it would cost far more than what you would save at the pump. So you keep the one you have until you have a bigger reason to replace it.