(Admin Note: Tomorrow I'll follow up on yesterday's post.)
GURPS has a lot of optional detail. One detail that's not especially optional is shields vs. bucklers. But for my own games, I don't split Shield into Shield (Shield) and Shield (Buckler). I find that too fiddly and nitpicky.
Instead, here is how I split it up for my DF game. Some of this is the actual RAW (rules as written) but some of it is a bit of a simplification.
Shield - a shield has straps, and is used by strapping it to the arm.
Ready Time - A shield takes 1 second to ready per (base, non-magically modified) DB.
Dropping the shield - Dropping a shield takes 1 second per (base) DB of the shield. This can be done with one arm, because I'm generous. In the meantime you get the DB but cannot Block as it is no longer ready. If you change your mind you can use one Ready action to get it fully back into action.
Critical Failures - On a critically failed Block, the shield becomes Unready. It cannot be used to Block, but the DB still applies normally. It can be re-readied in one second with a Ready action.
Buckler - a shield is held by a handle.
Ready Time - A buckler takes 1 second to ready, regardless of size.
Dropping A Buckler - Dropping a buckler is a free action.
Critical Failures - On a critically failed Block, a buckler is dropped. It is on the ground, and takes the normal amount of time to re-ready like any other weapon on the ground. Optionally, roll to see if it is face up or face down. Face down bucklers can be readied in 1 second with Quick Ready, or 2 seconds with a normal Ready. Face up bucklers take 3 seconds to ready, including 1 second to flip it over.
How does this work in play? - Very well. I don't, by the way, use the face up/face down rules. But otherwise, it's as written here. It works very well, and it's very fast. All you need is one skill (Shield, no specialties) and to know what the shield is and how it is held. Done and done. People choose shields, generally, because the extra time to get one ready is generally out of combat. But bucklers are faster in return for a risk of dropping it.