They may not be there to act as a grate, Im thinking they may have been added to improve material flow in the tooling. Large, thin walled, injection moulded parts require very high pressures to fill the tools evenly, these would certainly help with that. Since these parts are very low volume (woodworking parts etc, compared to almost any other commodity ) This would be a cheap, easy way to improve quality and add very little cost to the tooling like an additional fill point.If one wants a grate, they should buy a grate, not have one supplied, not in the description, for a Y .