Small bore 12mm telescopic gauges to measure 48 to 56mm, 

these are compatible with 40mm Elsa style boring bars with the two grub screw holes

simply close up the tele gauge and nip up the locking screw, slip the tele gauge into the tool hole in the bar, slide the bar into the end of the bore, use the supplied Allen key through the grub screw hole to unlock the locking screw and allow the gauge to spring out to the bore diameter, nip up the grub screw, remove the gauge from the bar and measure with your micrometer

the kit comes with two tele gauges 48 to 52mm and 52 to 56mm all packed in a robust weatherproof case 

Made in the UK

a demo is available here 

https://www.facebook.com/reel/1132790198971446