If, like me, you often turn vintage Fritz Hansen chairs upside down or even need a spare part, you may be surprised of the different attachments for the bases on the shell.
So here is a brief overview to clarify.

On this early base from the early 1950s no cap was used, the screw connection was open visible. The screws are inch thread.

The spacer was a simple round rubber disc and attached to the shell with a small screw.

In late 1950s Fritz Hansen started to cover the screw connection. First, metal cap was connected with 4 small nails, hard to remove if re-tightening of screws is required.

Also design of spacers first changed to a rectangular design (1.picture), later to triangular design which at the same time allowed the chairs to be stacked without damage. This design is, except material, the same till today.
Left picture shows still rectangular spacers, but already center screwed fixture.
The middle picture shows past version of spacers made from solid rubber, and current version made from plastic.
It is just pushed onto the leg and clamped under the seat, no glue, except when used on dot stool. Stool was glued at some versions.

In early 1960s Fritz Hansen screw connection changed to one screw in center connection. Metal cap and base was connected with one screw in center.
Picture 1 & 2 show a combination for 3-legged and 4-legged bases, picture 3 for 4-legged only.
Number on tag indicates month and year of production, in this case 1. & 2. picture December 1964 and 3.picture April 1966.

Unfortunately the base was not very strong and broke often.
Last picture shows a version for hall seating, it had no metal cap, just a center cross.

In late 1960s Fritz Hansen launched 3-screw connection on a round metal plate, covered with a plastic cap was introduced.
The number on cap indicates year of production. This one shows 1967.

In 2003, Fritz Hansen switched from a wooden mounting board to an ABS plastic board, which sticks poorly and coming loose. 
The base sometimes spontaneously detaches.

xxxxxxxxxx In late 1960s Fritz Hansen 3-screw connection on a round metal plate, covered with a plastic cap was introduced.
The cap indicates year of production. This one shows 1967

xxxxxxxxxx In late 1960s Fritz Hansen 3-screw connection on a round metal plate, covered with a plastic cap was introduced.
The cap indicates year of production. This one shows 1967