Keep in mind that while they called for specific engine models as Rick noted, the engine may have been changed along the way over the past 85 years, a common procedure to keep an old gal running and something we tinkerers still do today with a car or project we just can't seem to part with. On the driver's side of the block you should find a casting number and if in fact you have a W5 engine in the Rugby, it will be embossed there and shouldn't be too hard to find with a bit of rubbing. They did a good job embossing most of them.