Welcome to the forum !
A Siffert with a 142xxx makes for a very early execution. This would most likely be a ridged hour marker version with a Cal.11 movement and unpainted stainless hour and minute hands. These particular early executions are considered quite desirable & accordingly are not cheap.
A picture would help a lot to spot any errors or problems. Feel free to post a picture here on this thread or email me directly if you'd like advice.
Good luck - Darren

: Good Afternoon,

: About 6 months ago after collecting modern watches for 8 years, I
: decided to turn my attention to vintage watches. I have since
: learned that there is much to learn about vintage. I am
: interested in a particular Jo Siffert Autavia but the first 3
: numbers of the serial number on the case are 142. Would this
: case serial number be a correct serial number for a Siffert
: Autavia? Thank you so much.