first, i haven't been around for more than a few years (4 or 5 actually) - aggravating health issues have kept me busy, mainly.
This is for the 3813 with a 38" deck - But went to cut my grass, and the PTO belt blew - i didn't want to wait for one to ship in, so i took the old belt to NAPA - it's measured length (OD) is a hair over 106" but the replacement belts were wider and thicker from inside (surface to outside surface) so I brought home two, a B104 & B106. Those are industry standard belt size references, the B104 is 107" and the B106" is 109", price on either was $32.99. The longer length is required as the belt doesn't ride down into the pulley's groove as deeply
The B104 would not fit, the B106 slipped on fine and has been working fine - i've cut the yard (1.25 acres) twice now. One part i like is the belts are definitely more substantial, strength wise, in appearance, but the polymer could be weaker, i don't know. all i know is, $33 vs $50 (+ shipping) for the honda belt, NAPA is definitely cheaper.
If you don't live near a NAPA, McMaster-Carr has the belt for $28.40 https://www.mcmaster.com/#v-belts/=1dedfu3
Someone else had posted this same info years ago, but my google "fu" wasn't working, as i couldn't pull it up