[DCRM-L] long 12mo or 18mo?

[silvanan at mptek.net]

Hello there,
I was hoping someone could help me determine if my book format is long
12mo or 18mo. 
La liturgie, ou, Formulaire des prieres publiques ...
A Dublin : Chez G. Colles, R. Moncrieffe, C. Jenkin, & G. Hallhead,
MDCCLXXII [1777]
A-2C in alternating 12s and 6s, vertical chain-lines,
Can't really make out any watermarks.
approx. 13.5 x 8.5 cm
Most ref sources and records describe it as 12mo.

Any help would be appreciated.
Thanks,
Silvana