Every tekmar part and product has a date code. This should be located either on the outside label or directly on the product. Most newer date codes are in the format Mmm yyyy.
The date code is located in the bottom left of box labels.
Most controls have a small label located on the side of the printed decal. The date code is the top number. To see this in products such as a 256, 260 or 274, you need to remove the upper plastic cover.
For most thermostats, the date code is on a label that can only be seen when you open the cover and look on the inside. Small, 2 or 4 button thermostats can be opened by depressing a tab at the thermostat's top to release the catch, then rotating the front section forward. Larger thermostats with a flip out panel on the right have date codes on a label within the wiring chamber.
This is an example of a label that is located in the wiring chamber of a thermostat. The date code is located in the top right, underneath the product number, or the top left of the control cover underneath the lot number.