Hi Ken,
Welcome to AAPK!
It appears you have a Jackmaster #801. It should be 3-1/8” in length when closed. If you look at the ends of the knife on either side is a tab that holds that handle half in place. Each handle half is a piece of pressed sheet metal so that it appears to have a bolster and handles and some of them even appear to have rivets. The handle material is a piece of colored plastic wrapped around the handle half and held in place by being pinched between the handle half and the liner of the inner portion of the knife.
The inner portion of the knife functions perfectly well with both handle halves gone, indeed you often see them this way at flea markets. Inside are two springs, two blades and two liners, (one on either side, and sometimes they had a center liner). The pieces are all riveted together to form the inner portion of the knife, which is really the heart of the knife. The addition of the handles just makes it look more like a knife and definitely makes it more comfortable to use.
The blades are probably carbon steel as well as the shell handle, at least most of them I have seen are carbon steel. It is likely they made some in stainless steel prior to their closing.
Imperial bought the rights to a shell handle patent and begin producing shell handle knives circa the mid-1930s. They use the name Hammer Brand, which had been a high quality brand of knives, made by New York Knife Company. They did use good quality steel and they were able to sell these knives for a fraction of the price of a handmade slip joint knife.
The original concept of a shell handle knife was developed in Germany in the late 1800s and patented it around 1901 in Germany. I have attempted research for German shell handle patents prior to 1900, but it is nearly impossible to track them down, even with the assistance of a German knife expert. He explained to me that patent records were destroyed in World War I and World War II and he was unable to find copies of any patents.
Basically the design of your little knife originated in Germany, was modified somewhat and re-patented in the USA and used by Imperial. Imperial Knife Associated Companies was formed in 1947. It was a consortium between the Schrade Walden, Ulster Knife Company and Imperial International Corp.
In 1978 IKAC purchased the Jowika factory in Listowel Ireland, where your knife was made. Jowika had been one of the original German companies who made shell handle knives.
The Imperial Schrade Corporation factory in Listowel Ireland closed in 2003 about a year and a half before the bankruptcy and closing of Imperial Schrade Corporation in July 2004.
