I'm in the process of replacing the fuel tank after a slight leak (more of an ooze than a drip) from the old tank.

While the fairing and old tank are off I decided to check the fuel lines to find the they are dated 1985 . They've now gone and shiny new ones are fitted.

Under the airbox there was a manufacturing stamp on top of the engine block that also said late 1985 .  Several other date stamps on other components are from around the same time.

None of these would be an issue except the bike is recorded as first registered in August 1988 and, on checking the VIN is noted to be a February 1987 build.

Did BMW build bikes with 18 month old parts?

probably when the master positive cast was made ...not necessarily the pore date for the cast of your engine ,,although it could be correct ,,,and the parts not being assembled till the manufacture date ...ill guess its about the supply and demand chain to various markets ?

I have a few heads here with very similar casting marks ...although I know one comes from 84 ...one from 86  and another from 87- 88 sold to market 
 years

to me the earlier year  series casts appear better conformity  than later years