Thanks for your help.

old.ar15.com/legal/serialNumberList.asp

Text from the page:

Olympic Arms 360-459-7940


4-numbers only - very very old. Probably machined from bar stock.
1-letter and 4# - older receiver, but forgings. (Some are newer, POST ban castings - call to verify.)
2-letters & 4# - most receivers follow this pattern.
2#, 2 letters, 4# - the first two #'s are year of manufacture of the lower.
2 letters, 6# (separated 2 & 4 - e.g. SA 96 0000) - first 2 #'s are year of manufacture.
"ICR" marked lowers are an European customer’s overrun, has an unused additional hole - all ICR lowers are post ban.
Olympic Arms Manufacturer’s Notes:
1. There were some odd variations and some custom serial numbers mixed in thru the years also. And, some receivers manufactured after the ban have the year stamp after the model name, not in the serial #. And if you don't see a year stamp on the receiver, it is not necessarily pre-ban.

2. Serial numbers that end in "D" were duplicates and one was changed using this letter.

3. Recently some serial numbers have left with one or two letters followed by FIVE numbers. The last numbers are either 5's or 0's. 99.9% of these are post-ban.

4. "Older" does not always mean pre-ban as per Corey Sattler.

5. "BL" serial number series lowers are transitional with some pre and some post ban. If you have a "BL" you’ll have to call to verify for certain.

Go to www.olyarms.com

They have a Ser# page with all the dates of manufacture.