F1 use 304 stainless steel, M1 use 2cr13 stainless steel. M1 are with adjust bar (at the bottom of the weights), F1 are without. Feedback is very important to us.It is impossible to address issues if we do not know about them! Faction is our ultimate goal. We work hard to earn your trust and future business. A If you receive a defective item, you may return it or ask for a discount. Due to time zones, our reply to your questions may not be immediately. If it's non-workdays, please kindly be patient and wait till Monday. The item "25pcs 1mg -1kg Class F1 Calibration Scale Weight Kit Set 304 Stainless Steel" is in sale since Tuesday, May 14, 2019. This item is in the category "Business & Industrial\Healthcare, Lab & Dental\Medical/Lab Equipment Attachments & Accessories\Calibration Weights". The seller is "bestdealwow" and is located in Rowland Heights, California. This item can be shipped to United States.