On your photo from top to bottom. The top one appears to have no markings, which may be the early design that did not have a rear adjustment screw for the CO2 cylinder or it just could be missing. CO2 cylinder lengths and thickness varied among manufacturers. The rear adjustment screw allowed one to seat the CO2 cylinder just right for that particular brand. The middle one marked with SI, is equipped with the rear adjustment screw that looks wound all the way in. The bottom one also appears to have no markings, but has the rear adjustment screw that looks wound all the way out to the stop. Great sale of old school technology for Nelson based products.