Your clock was made by The Gilbert Clock Company, Winsted, CT. The F/S hole is a time regulator for adjusting the clock to run faster or slower with a key.
For a pendulum, you can use any pendulum bob from similar clocks - the weight is not critical. Of course an original Gilbert type replacement would be best if you ever want to sell your clock.
The pendulum bob is a circular metal weight a little larger and thicker than a silver dollar with a hook on the top to hang from the leader.
About Gilbert clock by: Anonymous
If you have the original key for the clock, the very small end, opposite the winding end is for the small hole marked s/f. That is to adjust the clock for faster or slower, if it is not keeping acurate time. I have a very similar clock with the entire label inside the back door. I purchased it last night at an auction for the rediculous price of $15. I would have gone much higher, but no one was bidding against me. I lubricated the clock, and it is keeping great time, and has a very pleasant chime.
Ed
