The Milestone shells -- each one lovingly hand formed by Clapham and Co. -- were made from a range of 'formulas', each one selected for tone and resonance. Type 2 and 4 made for great bass drums. Toms were available in formula 3 and formula 6. Snares came in formulas 5 and 8. To my ears, there are subtle but discernible differences between the formulas.