Look small pin cavity! New U-# or mismatched porcelain shells?
Here is Elton Gish's response dated April 19, 2015:
It is the Locke U-856 (5 x 4-3/4). The primary focus on selecting a U-number should be the profile, not the dimensions. The difference in diameter is perfectly acceptable for an insulator this size. It was made over a long period of time, so the chocolate brown glaze would indicate a late production. These were formed by hand to get the bowl form, so you can imagine there could be variability, but the profile matches U-856 very well. The wider lower section of the top indicates the bowl was not formed well to fit the more narrow lower bowl. This could explain much of the difference.