Don't know if you have already resolved this question. But, Sparcraft
3901 Pine Grove Circle, Charlotte, NC 28206 USA, Tel: 704-597-1502 should be able to help you. Don't know if David Lively is still there, but he helped me figure out which mast model is on my IP37 (1997) which was the first year they gave the option for the older models to have in-mast furling mains.
Mac (IP37 "Hamba Kahle")