Doesn’t look these clip-ons are legal in Kalifornia.
CA Penal Code § 468 (2017)
Any person who knowingly buys, sells, receives, disposes of, conceals, or has in his possession a sniperscope shall be guilty of a misdemeanor, punishable by a fine not to exceed one thousand dollars ($1,000) or by imprisonment in the county jail for not more than one year, or by both such fine and imprisonment.
As used in this section, sniperscope means any attachment, device or similar contrivance designed for or adaptable to use on a firearm which, through the use of a projected infrared light source and electronic telescope, enables the operator thereof to visually determine and locate the presence of objects during the nighttime.
This section shall not prohibit the authorized use or possession of such sniperscope by a member of the armed forces of the United States or by police officers, peace officers, or law enforcement officers authorized by the properly constituted authorities for the enforcement of law or ordinances; nor shall this section prohibit the use or possession of such sniperscope when used solely for scientific research or educational purposes.
(Added by Stats. 1958, 1st Ex. Sess., Ch. 76.)