I" Royal Enfield has produced the Thunderbird range of motorcycles, but there is no specific model called the Thunderbird F-16.
It's possible that you may be referring to a different motorcycle brand or model that offers both single-seat and dual-seat versions. Different motorcycle models often come in various configurations, including those with a single seat and others with a two-seat setup.
If you have any further information or clarification about the specific motorcycle you are referring to, I would be happy to try and assist you to the best of my abilities.