I don't think you're quite there yet.
I may have missed it in the thread, but: Was the car a "post/coupe", or a "hard top"?
You're good on the "RM
" being it was a Roadrunner.
Next, you need the Post or HT
designation - 21
respectively. If you don't remember: Did the back windows "roll down" (hard-top/23), or did they "pop-open-outward" at the back - a couple inches (post/21)?
You're missing the engine designation in you proposed number - (383-4bbl "H
Then the year "8
Then the assembly plant "A
" (Lynch Road per your trans number).
Here's where your number goes awry, though - pretty sure it is NOT 890051
: Sequential started at 100001, and Mopar built 547,507 total B-Bodies.... Belvederes/Satellites/Roadrunners/GTX's combined account for 241,573 of that (and 44,599 of those were Roadrunners)
My thought on this is (and I have never looked closely at the '68 numbering): It looks like (by the layout) that it was done intentionally backward - 890051A8
is actually 8A150098.
I think you are either RM21H8A150098