What is the difference between a 1000H and 1000H1 element?

The 1000H element is designed for use in a 7/8" line section while the 1000H1 is used in a 1 5/8" line section. Both H elements work across the 2-30 MHz frequency range.

An example of products that use a 7/8" line sections are the Model 43 family of wattmeters.