You might try visiting this website from Irwin Yachts. Irwinyachts.com. Should tell you what you want to know. I have a 1975 Irwin 33 ft myself. This website is where I got a lot of info about my boat. Good luck! Also, if you have the info on your roller furling you might see if you can get a measurement using a extra halyard and compare that to what the info on your furler says. Dont know if that will help or work but maybe someone else here can chime in with some more info. I saw your other post about the mast bend on your boat. I have a furler on mine but I dont know if it would be the same or not.
1975 Irwin 33