Size Tips:

For Fit 31.8 33.9mm Flared option: it fit Brompton 31.8mm Flared Seatpost, 31.8mm seatpost and 33.9mm Seatpost.

For Fit 31.8mm option: it fit 31.8mm seatpost, it don't fit Brompton 31.8mm Flared Seatpost.

For Fit 33.9mm Flared option: it fit Brompton 31.8mm Flared Seatpost and 33.9mm Seatpost.

1. the plug is suit for 31.8mm diameter or 33.9mm diameter seatpost,so plug diameter will smaller than 31.8mm or 33.9mm.please understand.

2. the rubber plug is the universal model with stepped design. Once installed, it not only protects the bottom of the seat tube, it also effectively prevents the seat tube from being pulled out completely.
About it:

When we fold the bicycle, the seatpost easily touches the ground, causing wear. This product can effectively protect the seatpost.

the rubber plug is the universal model with stepped design. Once installed, it not only protects the bottom of the seat tube, it also effectively prevents the seat tube from being pulled out completely.
Product Details:

Name: Bicycle Seat Post End stoper
Series: British flag series
Fit: For Brompton, SIXTY, PIKES, CAMP, Dahon, Fnhon, Tern, Crius, Royale Seatpost 31.8mm, 33.9mm
Material: Rubber, plastic

The rubber plug is small at the top and big at the bottom, please check this picture. Depending on the size of seatpost, the final effect will be different. Some 31.8mm seatpost do not fit fully in. it does not affect the use and this is normal.

TWTOPSE Bike Seatpost Plug 31.8 33.9mm For Brompton Folding Bicycle Seat Post End Pad Protector Seatpost Nipple 3SIXTY PIKES