This N15CS originally had Monobloc's and used the 1/2 aluminum thin spacers p/n 030561, with the balance unions. So these stayed the same regardless of type of carb installed on the N15's.
Side note:
Bill, I ran the number 23358 on the ANIL site, shows a fibre spacer and listed as 30mm for intake bore. Other numbers were not found.
I looked in my book Norton Spare Parts list 1963- 1965 Models 88, 650SS, 650/99, & Atlas, it lists in the Carburetter section,
23358 - spacer carb - thin qty 2 ALL so this would be fibre
23872 - spacer - carb - thick qty 2 88ss, 650 and Atlas, material unknown at this time. Most likely fibre. In the section for 1966-1967, 1968 this spacer is call out for 650SS, Atlas 1966
030561 - spacer carb - ALL G15 aluminum 1/2" thick. Balance unions installed on this part.
My 1965 Atlas which has retrofitted concentrics, has a 1" aluminum spacer with balance unions, and a thin fibre spacer. Wonder what this thick aluminum p/n is?