I just used my Oster blender from the late '60s. Works perfectly. There used to be mason jars that fit. I also have a set of very small jars with a yellow Oster label painted on them, original.

I have the Oster Kitchen center that has the blender attachment. It's also from the late 60's or early 70's. I also have the small jars like yours - still in the original boxes. I haven't tried using Mason jars but I have one I'm going to try.

I bought an inexpensive Oster blender (DuraLast Classic) not too long ago. As Tothill so well described, the blender ring fits on the top of a regular mason jar as well.

On another note, the Laundry sauce recipe is similar to one I have made, but as a powder. I shred the bar soap with a grater, then mix in the Borax and washing soda and use a tablespoon in my wash. I'm going to give your method a try..grating the soap can be hard on the arm/shoulder. The powder method does not work well in cold water as the soap needs warm water to dissolve. That problem is solved with the use of boiling water. Let us know how you like this soap.

Some of the older blender you can turn the mason jar upsidedown and screw it into the base. I'll check mine out upstairs as I've had mine since 1968 and it still works like a charm. Get back with you later with the results.

How do you keep the stuff inside the jar if you turn it upside down? I guess I need to watch a video.

At the end of Day 2 she says it looks like Mayo, so it is thick.
You remove the whole blade assembly off the blender like you would lift off the pitcher to pour.

Like Chasing Hawk says, remove the jar along with the blade assembly still attached. Then invert with the blade assembly on top and the jar upright. Unscrew the blade assembly and the contents remain in the upright jar. Easy peasy.

So many uses for canning jars
 

I hate to throw anything away but I am getting old and don't want to leave all this stuff. I have most of a "Kitchen Center" with five appliances with one base. Or I have a lot of parts, some new for a broken CPAP machine. Too bad we are all so far apart. All my young years I bought fabric at yard sales and store sales so in my old age I would have fabric and could make quilts even if I couldn't afford fabric. Now, in my old age, I have no time and poor health. I asked the price of an item at a yard sale and the seller said "what is it?" In the olden days farmers had glass bases that if one put a canning jar, full of water upside down and it provided water for chickens. I think anyone over 50 should write a book because life was so different that today's youth could not even imagine life as we knew it. Thanks to all of you for the laughs and shared experiences.