A picture is worth 1000 words or more in this case. What's "special" can be subtle to collectors.
I don't know of any early Colt AR-15s with just a 4 digit serial number. If it's marked SP018xx then it was made in 1964 and would be a collectors item as only about 2500 were made that year. Does the lower have a factory name engraved? AR-15s of that vintage didn't have forward assist buttons.
You might also ask the same question (with photos) on
AR15.Com - Your Firearm Resource. (AR-15, AR-10, M4 Carbine, M16, H&K, SIG, FNH, FAL, AK-47, 50 Cal, M1/M1A, Handgun, Pistol, Training, Hunting, and More!) where there are some knowledgeable collectors.
Value? More than you could buy a new one for if it's in excellent condition.