It seems you're referring to a "double-sided key" or "double-bit key" used for locks. The numbers you mentioned, 21mm and 23mm, likely correspond to the length of the two "bits" or sides of the key.
A double-sided key typically has two sets of teeth or notches on opposite sides of the key. The key is inserted into a lock with the appropriate orientation, and the teeth on both sides of the key align with corresponding pins or tumblers inside the lock to allow it to turn and unlock the mechanism.
The specific measurements you provided, 21mm and 23mm, would likely be the lengths of the two sides of the key, which can vary depending on the brand and type of lock for which the key is intended.
If you have a key with these measurements, it's important to use it with the corresponding lock it was designed for. If you have more specific questions about a particular lock or key, please provide additional details, and I'll do my best to assist you.
