Tektronics 453 - Saving a single shot waveform for later analysis
Camera.  The word you are looking for is camera.
Tektronix made one for your scope, but good luck finding one these days.  You'll need even more luck (and maybe deep pockets) to get the needed instant film for the camera.
This is the camera Tektronix made for your scope:


Shameless plug
As an alternative, I can suggest the software I wrote to do similar tasks with my equally old Telequipment D43.
You'll need a web camera and an adapter for your scope.
Mine looks like this:

The software can capture one shot events, despite not having any electrical connection to the scope.
This image is a one shot capturing contact bounce on a switch:

You'll notice there are cursors on screen, and readouts below - you can make measurements like on a (somewhat) modern digital storage oscilloscope with the software.
You can also digitize the trace.  That only works for a single trace.  I use it to export data to other programs (like Baudline for spectrum analysis.)
I have a bunch of blog posts on it, and there's a Wiki for it on GitHub.
The software runs on Linux and Windows, and probably on Apple (if some one could try it out.)

As a final alternative, you could try the Fads to Obsessions webcam oscilloscope software.
It looks like this:

I tried to use it, but couldn't get it to run under Linux. (It is admittedly a Windows only program, but the Wine Windows emulator can some times work wonders - except this time.)
It's just as well that it didn't work.  My software runs real time - the Fads to Obsessions software seems to only take snapshots on demand.
It has a different set of features than my software.  Maybe it will suit you better.