I have compiled a list of the different material codes that Seiko have used over the years. These case codes describe the material used in the construction of the watch case and are listed in the JDM catalogs as well as noted on the back of some cases.
For example the case material codes from the 1991 Vol. 1 Catalog, e.g. SBBS018 has the case material code of THGMCETIHG, the SBBN001 has a code of TIHC.TI and the SSBT014 has TIHNT.TI. These are all Titanium cases but have different finishes.
You can see the list of codes here - Seiko Case Material Codes