Business Source Window Envelopes, No 10., Side Seam, 4-1/8" x 9-1/2", White (BSN42251) Pack of 500No. 10 window envelopes feature an economical side seam and a window with film cover. Oyster white, corrugated, expandable box provides easy access to envelopes. 24 lb. General Information Manufacturer: Business Source Manufacturer Part Number: 42251 Brand Name: Business Source Product Name: Economical No. 10 Window Envelope Marketing Information: No. 10 window envelopes feature an economical side seam and a window with film cover. Oyster white,