Two things about this: Whether the Fifth Amendment remains available here may turn on what he’s already said, either publicly or in testimony to a friendly HPSCI. To the extent he’s already talked about specific correspondence or documents, he may have waived privilege ...
Second, and more importantly, the Fifth Amendment only applies to the production of documents when *the production itself*—the fact that the person has access to the material described—would be “testimonial” and potentially incriminating.
That is, you normally can’t invoke the 5th to avoid a subpoena for specific documents, even if the *contents* of the documents would incriminate you, unless *the fact that you even have the documents* would be incriminating. (“Produce your original copy of the ransom letter...”)
Of course, you don’t have to actually be guilty of anything to plead the 5th, but it does have to be the case that the production would “tend to incriminate”—that is, plausibly create an appearance of guilt, even if it’s a false one.
Formally, this may be a legitimate claim given the letter’s objection to the broad language of a request for “documents concerning” certain matters. To pick an absurd extreme, imagine a demand for all correspondence related to a criminal conspiracy.
Production there is pretty obviously testimonial, because you’re requiring the recipient to use their own knowledge to pick out the messages responsive to the request for you. You’re conscripting them to add info that may not even be apparent from the produced document itself.
If a subpoena asked for messages concerning hacked DNC e-mails, and you have texts where these are referred to as “the material” or something, producing them would add information from your brain (knowledge of what you were talking about) to what’s available on the page.
THAT SAID... yeah, it sure doesn’t look great that his response here amounts to: “Admitting I have responsive documents would itself look incriminating”. (Someone who had no responsive records would probably just say so.)
Also, the letter conflates two pretty distinct claims. One is essentially an overbreadth argument. The other is the 5th Amdt argument. They’re distinct and probably wouldn’t cover the same material.
Finally: depending on the precise wording of the request, this refusal may be at least partly valid, but that may be something the Senate can remedy just by modifying their request.
If they narrow the request to, say, specific people Stone is already known to have communicated with rather than subject matter, I very much doubt that either the overbreadth or 5th Amdt objections would hold up... IF the majority were willing to enforce a subpoena.
Which is, of course, the million dollar question: Is the GOP majority prepared to make Feinstein’s “request” to Stone compulsory, and then litigate the objections if necessary?
I’m doubtful, though they’ve certainly been less nakedly hackish about this than the Nunes HPSCI. But it may also be moot, since I’ll spray-tan myself orange if Mueller doesn’t already have a boatload of Stone’s digital correspondence.
Though there are very likely encrypted communications they’d only be able to obtain in plaintext directly from either Stone’s devices or the person on the other end of the line.
I guess that was more than two things...
