Just an update and question. First, thanks to Frodo's identification Delta has replaced the 72172 under warranty but interestingly the new version of this model at least in Canada (below) is all metal.
Second, I installed the new one and I guess I'm not familiar with this type. The stopper does not quite go down enough to engage the popup unless I either unscrew the stopper a bit or slightly unscrew the brass part it screws into, but loosening either causes a bit of wobble. Am I doing something wrong?