The 4 digit data code represents the year (first 2 digits) and the production week (second 2 digits). It is printed on the tube and should read like this: 9811 where 98 is the year and 11 is the week it was made in. Other Eimac tubes using the same date code scheme.

Also remember that the use "getter" so before using after long storage (or lack of use) always turn them on in stand by for a while (several hours to even several days) so the getter can scavange the last molecules of air from the tubes.

Any Eimac 8877 with date codes between 8608 and 8905 are likely to have poor reliability if they haven't failed already.