I know most glass people don't care much and many mudders don't look at this album but what this represents is the duration of production with virtually no change in design. The crude off white U-180 on the left is surely one of the very early examples while the one on the right is the last of the breed and despite having so many of these and having seen and handled many more, this is the only one I've ever seen with the skirt recess embossing. Not sure how many years are between these two but it was obviously a successful design that had a nice run.