Army - Cavalry Regiment - Guidon - Red Blue X 300 Classic License Plate

The user's query refers to the 300th Cavalry Regiment guidon, which features a red and blue X (saltire) design. This specific design is non-standard for US Cavalry guidons, which typically were red over white swallow-tailed flags, or later, a blue field with stars. The user's query likely refers to a specific, potentially unofficial, historical, or unit-specific guidon that is not part of the general U.S. Army regulations. 

The "300 history" part of the query and the unique design suggest this may be a highly specific, possibly non-US or a specialized unofficial unit flag. No standard U.S. Cavalry Regiment, such as the 1st, 2nd, 7th, or 10th Cavalry, has historically used a red and blue saltire guidon as a primary, officially recognized standard.