I emailed Corima regarding a wheelset I recently acquired; their advice for my rim (19.2mm width) was 23c max and they advised against trying to glue a 25.
thanks, i have similar feeling about mine so decided to put 21C there and use it as training wheels on track. for road use i'll build another set. i'm lucky to have few nos old rim sets, perfect for everyday riding.
last try, 24mm tubs for 18mm external width rims (nos campag omega strada v)?
i wonder if it's also 19-25 as it's with clinchers:
http://velobase.com/CatalogScans/Campagnolo/Campy_1991Rims.pdf