The closet flange gasket kit is part #12524 for the Aqua Magic IV & V.
Your toilet (31661) is the 'V'.
The kit runs about $10---$15 depending on where you order it from.....
LINKThere is also a Thetford Universal closet flange seal that fits all Thetford RV toilets for about $5.....
LINK
Is it time for your medication or mine?
2007 DODGE 3500 QC SRW 5.9L CTD In-Bed 'quiet gen'
2007 HitchHiker II 32.5 UKTG 2000W Xantex Inverter
US NAVY------USS Decatur DDG31