U-928 and U-928a Thomas Boch patent insulator:

Here are three examples of Boch patent insulators from left to right (All made by Thomas):

• Dry process version - This is probably the first type made to the Boch patent.
• Brown Glaze - Most of the U-928 styles are white but a few were found with this brown glaze.
• White U-928a - This larger version is much less common than the U-928

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