Wonderful table....really very very nice!!
I would put it at much closer to 1880 than 1930.

You have found quite a bit of time line information on Jannsen. He arrived to Clinton, Iowa in 1864, therefore, the date of the table most likely is after 1864 or maybe 1865. Presumably, he needed some time to establish his shop after arriving. This is an interesting date for an immigrant to arrive in the mid-west. The Civil War (1860 - 1865) was in its final days in 1864 and the country was torn apart. The infrastructure of Iowa was not as affected as southern Missouri.

Thanks to both lindac92 and jemdandy for your comments. I was hoping that the style of the table might nail down the time it was built. Your comments have forced me to think more about the timeline. Janssen would have been 34 years old when he arrived in Clinton in 1864, so his "prime" furniture making was probably over the next 20-30 years. Maybe this is as close as I can get, assuming his sons did not build the table at a later date. My goal is to place a brass plate on the underside of the table with the key information on the history. This walnut is so heavy and dense, that it will likely last another 100 years with care. One of my kids will surely want it!

Another 100 years??...Another 300 years! Look at the size of those boards in the top! Iowa was noted for it's walnut trees and walnut lumber. I have a couple of piece with those huge wide boards from a giant tree!
Assuming that when Janssen came to Clinton he didn't buy a stock of already dry walnut, that wood had to be cut at his sawmill from the trees on his property. Since walnut wood need to be VERY dry before using, it would be a good estimate that that table was made about 1880....give or take7 or 8 years. Also, that style was very typical for that time.
BUT...it could have been made from lumber from the site that had been stored at a much later time....not likely....but possible.

Again....it's a wonderful table!!!

Thanks again! Yes, I probably low-balled it at another 100 years. I like you reasoning on the time line. I had not considered the milling and drying of the walnut.

I agree......don't let the original casters escape....even if they do scratch your floor.

The similar one I restored last year really needed the casters to get to a comfortable height, the apron/frame is so deep it hits your knees.

The one I had in the shop was clearly factory-made with obvious low regard for quality control, and had a patent cast iron mechanism for the drop leaves; it would not surprise me if yours was sort of a local copy made in a rural shop where they did not have that hardware available.

It seems 1880's to me.

Oval drop-leaf extension tables are really neat, until I saw the one I restored I did not know they existed. Mine had five leaves, opened to over 10 ft, but with the drops lowered, was about 34" long. Crazy cool design.

Casey

The patented table support thingies were a later innovation....like after 1900. The older tables had wither a wooden turn around in the apron to support the leaf, or a pull out like the OP's. Most of the ones with the iron leaf support that I have seen were oak....telling me they date to the "oak period">
I have a love affair with walnut drop leaf tables...I used one as my diningroom table for about 25 years. It had 5 leaves and opened out to seat a bunch, but with the leaves stored and the sides down it was 32 by 29 and worked well for a small space. I gave it to my daughter when my mother died and I inherited her diningroom table. And I bought another oval walnut drop leaf that opened for leaves for my son when he married, and I bought a 36 inch square walnut drop leaf for my kitchen, and another larger one rectangular, about 40 by 52 or so, and another smaller end table with 2 drawers and leaves that flip up, and another square walnut one that I sold to a friend for a song....because I didn't need it. All have had the wooden turn around leaf support but for one that had a pull out like the OP's table.
I love walnut drop leaf tables!!!
If you sit at the short sides, you really need the casters....but if you sit where the leaves are or at the ends, there is more room. interesting, the rectangular tables never had casters, the oval ones did.

Is OP "original poster"? With the drop leaves up, but no addition leaves, the oval table is 44" wide and 52" long. With all 3 leaves inserted it is 94" long. Agree on the apron/frame hitting your knees on the side. The new ball casters that I installed raise the table another inch, which really helps. I will find a place to store the original casters with the table.

I also love these walnut oval drop leaf tables. I refinished another one that was my grandma's a few years ago and gave it to my daughter. It is a few inches narrower and only has 2 leaves. The drop leaves have an iron piece that "clicks" in place when you raise the drop leaf. To lower, you reach under and pull the iron rod down to release. Definitely an old mechanism.

When I said the caster raise the table "another inch," I meant over the previous height with the original casters.