It seems complicated but it isn't really. The 8th character in the VIN is represented by an 'alpha' character (H, J, K, L) depending on the model year. This letter will correspond to the model name that Honda uses to designate which model the bike is. Usually the lower model name represented by an IV or 4 will be the basic audio comfort model and the higher the roman numeral or number will indicate more features such as NAV, XM, ABS or AIRBAG. The Factory Service Manual lists all the types of models for the model year covered by the manual. For instance, my Factory Service Manual covers 2012 and 2013 year models. The table on page 1-3 lists all the model types and what feature(s) are included on that model type.
IV or 4 is the Audio Comfort model
V or 5 is the NAV XM model
VII or 7 is the NAV XM ABS model
VIII or 8 is the NAV XM ABS AIRBAG
THere is also a suffix, either an 'A' or 'CM' following the number which indicates the destination of the bike.
A = USA
CM = Canada
The higher model code includes all the features of the lower code(s).
Any Honda dealer should be able to decipher by the VIN number which model you have. Some online parts sources such as hondadirectline.com will list the various models by year broken down by VIN number which correlates to the particular model of the GL1800. The parts diagrams and the service procedures in the FSM refer to the model by IV-A, IV-CM, V-A, V-CM, VII-A, VII-CM or VIII-A or VIII-CM
Confused yet?
.
Hope this helps.