Starting at the beginning, all Colt commercial AR rifles appear to have had an "SP" prefix serial number.
The "51" is simply the first numbers of the serial number.
I seem to recall that Colt stamps the first two numbers of the serial with the SP and that allows setting up the roll stamp to only have to change for the last digits for almost 10,000 rifles.
There may be a slight off-set of the SP51 and the rest of the numbers.

 

I too heard the SP is used on all commercial, and was told it was used to indicate "sporter"

 

What about if it begins with "BK"

 

No idea.
The available Colt serial number references stop in 1985 and they show only "SP" prefixes for the AR.
Possibly "BK" is either a later prefix or it's a prefix used on a police model.

You can call Colt during business hours and they'll give you a production date. They won't give more then the date unless you buy a Colt Archive letter.

 

Thanks. I think i seen elsewhere they say "BK" may be a post ban era number

 

BK = Banned in Kalifornia"?

 

I believe the different prefixes were used on different series of firearms. SP is for sporter, MT match target, SL sporter lightweight, ST sporter Target, GC government carbine, MH match heavy. There are lots more, and the numbered portion will determine the date of manufacture, some pre ban, some post ban. I am not an expert, and have been incorrect before.