Health Insurance Pres-Stik HCFA Envelopes 3880, 4-1/2 x 9-1/2", Self Seal, Box of 500. White side seam windows with blue Wesco tint. No. 10 1/2 envelopes are designed for use with Insurance Form CMS-1500 and similar forms. The 1-1/8" x 4-3/4" window positioned 3-7/8" from the left and 1-1/2" from the bottom of the envelope. Inside security tint offers more privacy. Self Seal Flaps ( No Moistening Required) 24 lb.