What is missing from this discussion on UNC vs BSW compatibility is Class of Fit which will be a higher class for auto/motorcycle engine parts, and low for tripod screws and sockets for ease of mounting. The low Class of Fit will permit interchangeability between UNC and BSW tripod screws and sockets for limited thread length. The camera and tripod fittings were standardized in the mid 60's to 1/4-20 and 3/8-16 UNC. ISO 1222:2010 applies. The Calumet is probably UNC. My Tiltall (purchased in 1990) is UNC. I concur with the recommendation to drill and tap for 3/8-16 and insert one of the available 3/8 to 1/4 reducers. A bottoming tap set may be needed. An alternative repair is to make a flat plate that matches the Calumet's base footprint, drill and tap the 1/4-20 threads in the center of the plate, and mount the plate to the base with multiple flat head machine screws.