Though generally called a square collar, this very useful part will normally be used as a Bearing Block for standard Meccano Axles.
Made from Solid Brass 1/2" square and 5/16" thick (12mm x 7mm) 4.1 mm bore and 4 threaded holes on sides.
Threads can be Meccano 5 /32" or 4mm (m4), please choose option 
Made in the UK