These are all different glass electrode housings for neon lights. They are based on a patent from Samual C. Miller 2,046,960 July 7,1936. Top row is front of embossings and the bottom row is the backs. From L-R- Pyrex No.100 clear, P-K Industries No 100 clear,(Peterson P K Industries Berkley CA.), P-K Industries No. 100 (same embossing green tint glass), Standard No. 100 clear, Top porcelain cap Milhan 20 (American Sign Co.Kalamazoo, MI.). This came off of the RCA radio building sign in New York., Peterson P-K Industries No.200, green tint glass, Westrim P-K (these were sold by Westrim Products Co. France and P-K Industries but made in China). It uses a metal cover to screw on the glass and attach to the sign.