Opto-coupler / Opto-isolator made by Silonex. Consists of a yellow LED and a single CdS Photocell in a hermetically sealed package. A great item for experimenters and automation proto-typers. Also has applications in audio switching and music effects devices. Resistance of photocell measured between 1500 and 2100 Ohms when powering the LED. Off resistance measured greater than 6 MegOhms. Physically similar to NSL-32 coupler. 

• Device Type: Optical Coupler (LED Photo-resistive-cell)
• Case: 4 leads, total length of unit approx. 2 inches.
• Semiconductor Material: Ga, CdS / Not applicable
• Specification: Not available (see description above)
• Markings: Dot to note LED polarity. NSL-7053 embossed on case (this is likely the part number of the plastic shell and not the unit part number) 
• Company Website: advancedphotonix.com/optosolutions/
• Source: Inventory Surplus