The stamping of the serial numbers isn't always easy for me to read. I am wondering if the alpha on your serial number is a "G" rather than a "C"? If yes, here is the information I found on ismacs:
[TABLE]
[TR]
[TD]LETTER
G-[/TD]
[TD]REGISTER NMBR FROM
5862331[/TD]
[TD]REGISTER NMBR TO
5912330[/TD]
[TD]MACHINE CLASS
(MODEL NO.)
15[/TD]
[TD]QUANTITY ALLOTTED
50000[/TD]
[TD]DATE ALLOTTED
January 3[/TD]
[TD]YEAR ALLOTTED
1918[/TD]
[/TR]
[/TABLE]
Below is a picture of the serial number on my machine. I read mine as a "G". I pasted the ismacs info for this serial number too showing that my machine is a 115:
[TABLE]
[TR]
[TD]G-[/TD]
[TD]4663716[/TD]
[TD]4673715[/TD]
[TD]115[/TD]
[TD]10000[/TD]
[TD]May 3[/TD]
[TD]1916[/TD]
[/TR]
[/TABLE]