Let me try to explain.
Your seatpost clamp is offset. Meaning that the clamp mechanism is behind the seatpost. Typical of a single bolt clamp.
With a straight Thomson, the clamp mechanism is inline with the post because it's a double bolt clamp. It grabs the seat rails above the post and there is a bolt in front and behind the post.
If you like your seat position with your offset post, then buy Thomson lay-back post which has a bend in the top 4 inches of the post. This put the seat in the same relative position as your existing post.
Last thing. Yes they are expensive, but worth every penny!
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