Thanks for your intervention Azonic, in effect the problem that you analize is well known. The reason for the decision to don't make that holes is mainly structural: the shoes are made to be as strong as possible and the holes in that position makes the middle section a little too thin. This is the same reason why i have choosen to don't make the holes that pass through the flank of the chain. Instead i have changed a little the design of the chain links in another zone to minimize the problem that you notice: if you carefully watch the point in which one shoe is coupling and slide with the following one (in the bend the chain makes around a wheel) you can see that there is an opening that (i hope) helps the dirt to not stuck in. In the past set of photos this detail is not so simply to observe but in the next you will clearly see it. Other further solution to the problem could be the lowering of the profile of the teeth on the sprocket.
Finally ther is a thing to say about this problem: the typical weight of this reproduction models is so low to amplificate a lot some limits in the functionality of the chain-shoes, so it seems to be a must to provide to a spring that makes the freewheel slides back to prevent the lock of the chain in the case a little stone is stucked in between.
I hope to have made the right choises and the chain works well, but i'll can say that only after some tests.
Any other constructive critical intervention from you and each others will be greatly appreciated.....
Bye!
(sorry everyone for my english, i'm not a mothertongue like you can see)