I'm giving my age away, but I used to have hundreds of 45 rpm records,
and an equal number of those little gizmos that fit inside the hole of
the 45! When I was growing up in the southern U.S. in the 1950s, my
friends and I used to call these devices "spiders." When I was in
college in the 1960s, we called them "Triskelions," after an episode
of "Star Trek" which featured a design that resembled the little
In case you're curious, this is a Triskelion, as shown in "Star Trek":
Star Trek Dimension: The Gamesters of Triskelion
And, for the very young, here is an image of the 45 rpm hole-filler in question:
On the Web you'll see these doohickeys called by many different names:
"45 inserts," "45 adapters," "spacer disks," "45 single adapters,"
"45 rpm record inserts," and on and on.
This comes from a site called 45s.com, a great place to buy vinyl 45s:
"Q. What is the little plastic insert called that goes in the
middle of a 45-rpm record so it will play on a turntable?
A. It's called a 45-rpm adapter. (This is the question we are
asked more than any other.)"
If 45s.com says so, I think that's definitive. My final answer: 45-rpm adapter.
Incidentally, you can still buy the little yellow plastic whatsits.
They cost $4.75 for a set of 25:
Baby Boomer Headquarters
Or, for $299.95, you can get a 45 rpm adapter made of 14 karat gold:
The Hackensack Record King: 45 RPM Adapter Pendant
It should be noted that there was a different type of device which
went on the record changer's spindle rather than in the hole of the
record. This was called a "45 rpm automatic spindle adaptor." Here's
an image that shows a collection of these. And, in the lower right
corner, of the image... the ubiquitous 45 rpm adapters, here called
Google search strategy:
Google Web Search: "45 rpm" + "insert OR adapter OR adaptor"
Thanks for a fun question! If anything is unclear, or if a link
doesn't function, please request clarification; I'll be glad to offer
further assistance before you rate my answer.