Top shot of firing rests. The rust/mustard is definitely a NN THOMAS U-939B but the white example is 1/2 inch or little more shorter, much thinner porcelain shell and base section. Looks older with lots of black specks impurities in the porcelain that were glazed over. Is the white one simply a very much shorter than normal NN THOMAS U-939B or? Any help appreciated. Thank you!