Our ivory Jotul 3CB was manufactured in 1998 I believe. We are the second owners and have owned it for about 12 years. When we purchased it it was already set up for rear flue. We have since moved and would like to use it with a top flue - after reading my manual I see on page 4 that it states "The 3 CB-Ivory may be vented from the rear only - the cast knockout has already been removed." Anyone know why?