From knowing you for a few years on another sewing machine forum, I know that you have a lot of sewing machine knowledge, so I’m sure that this one slip will soon be overlooked.
Now, back to the block, I’m really glad that I brought up this discussion, because I have never seen a Singer 401A "lower receptacle extension block" like the one that you pictured! All that I’ve ever seen are “straight-through” blocks that exit out the end of the case base. Thanks for showing me something new!
CD in Oklahoma