There are only a few threads in the lever body, so a bit tricky to measure the diameter and pitch. Typically 3/16″x 26tpi, or 2BA. If you have sets of Cycle Thread and BA taps and dies it may be possible to use them very gently in your fingers as a thread gauge. Unlikely to be BSF but also worth a try. I have hundreds of small bolts, screws, set screws, etc., so often end up working my way through them until I find some that are a nice snug fit in an ‘unknown’ item, where I can’t get a thread gauge in. If the threads are badly worn it may be best to drill them out and tap an oversize known thread. A bit of a b—r when you have just had a part nickel plated !