The Rock-ola Personalized Extrusion # 41515 is used in model 426.
It is shown on pages 2 and 3 of the model 426 Parts Catalog as Item 23, "Personalized Extrusion # 41515".
It nearly the same length in the 425 but in that model they specify # 39821 which is about 27.25 inches in length. 

The 426 came with a Personalized Lettering Kit # 41538 that had letters and numbers which are 1.625 inches tall.

This item is 27 inches long by 1.875 inches wide.