Steve,

I too have a WP Improved Seneca, and have spoken to Paul, Oren and others involved in helping with the specs on the WP holders. The exterior dimensions of the holders should fit just fine. The T-distance on the Fotoman holders will be .260 inch. The old holders generally measure around .254 inch, but various samples by different manufacturers ranged from .254 to .257 inch. So, it may be that the new holders range from .006 to .003 inch away from the spec on the new holders. Should be good for contact printing.