As said, there is a conical base for many inches of any tusk. It is called the root socket. If the tusk was broken off, it might be missing.
It can be 6" or so deep on a walrus tusk and a foot deep on an elephant tusk. Hippo and warthog teeth have a 1-2" socket. On an elephant tusk, the nerve channel continues down the tusk a good ways as a small hole. Walrus does not have this hole.
There is something odd looking about that stub. None of the tusks I have had looked like that ( could be the lighting?). It does look old, and would probably be called a "fossil" tusk. I would suggest you cut the stub off about 3/4" from the end and post a photo of the straight and sanded cut. That will answer all questions. ( you can make a belt buckle from the stub cut-off)
If it has a tapioca looking core inside solid dentin - walrus
If it has cross-hatch lines and a nerve hole in the center - elephant
If it is solid and chalky white or crystalline - fake
If it cuts on the band saw with difficulty - fossil - 100 years or older
If it cuts like really hard bone - newer - 1 to 100 years
If it doesn't stink while cutting - fake
Since you are already in the USA, and unless you are planning on shipping any knife you make to another country, there is no problem with using this tusk for knife handles. Don't take it to Alaska, as they have state law issues on who can work tusk. Even Washington state can be a problem for any newer looking tusk. California can have rogue game wardens who take it upon themselves to write new laws, but have no authority to confiscate anything.
Yours appears to be old, so there would be no problem in any case.
Print a sheet with the photos you have posted, and write a short description of how the tusk was found. Sign it and date it.
That is enough provenance according to the USFW and other agents I have talked with. Give a copy of this provenance sheet to anyone who buys a knife with this walrus. It really isn't necessary, but makes a great sales tool. Having that provenance with you if an USFW agent asks will take care of any inquiry at a knife show.
Email me for more info on this subject, as I have some Government reference material and communications on the subject that I can send you.
sapelt@cox.net