That is the size but...Fritz was having a bad writing day....it is 56 1/2 not 66. The largest helme size in my experience is a 60 cm. The smallest 53 cm which is like a pin head. I once owned an M15 JzP stamped size 60 cm (rear rim). I have a Prussian OR M95 marked 53. I am working on a Prussian Garde officer...size 53 which is the smallest I have ever seen in an officer helme, and that really creates problems restoration wise! The Saxon JR 133 listed in the restoration section is a size 54. Sizes on officer helmes are always written in blue or black on the inside of the shell. Some have the size marked on the button which secures the end of the spine and they do come in half sizes. I do not believe that govt issued helmets came in half sizes. In my experience any half sized helme is private purchase. Sizes on govt issued helmets can be ink stamped or punched using a stamping tool. The size can be stamped on the inside of the shell or on the rear visor. The Saxe 133/34 in the restoration thread is a punched 54 cm. I have found that Bavarian Regts tended to punch their Regt information on the rear visor but again...... anything goes, it seems, there were no rules!
In my experience, the JzP and Kurassier OR helmes are size punched on the inside back rim of the lobster tail. Pre war helmes and even war time helmets of these types may also have the Regt numbers punched in this area. Then of course, we may find information stamped under the kokarde on these metal helmes. This has given me an idea for a new section on the forum dedicated to the metal cavalry JzP and Kurassier helmes.......to be continued!