Los Alamos recently had an unclassified display commemorating 30 years since the last US nuclear test. I sent the LANL library an email asking for photos of the display and in it they included images of the Julin Divider nuclear device. Divider being the last US nuclear device fired. To the best of my knowledge, this is the only public image of a US test device from the days of underground testing.
I made the images available on Wikipedia:
https://commons.wikimedia.org/wiki/Category:Divider_(nuclear_test)
I believe that this device is the W91/SRAM-T warhead. This is based on the device being the
same colour and shape as the SRAM-II/SRAM-T nosecone (Divider was a LANL test, which rules out the LLNL developed W89 SRAM-II warhead). It even has the ring of paint. At a glance, the material also appears to be the same. The tip of the warhead section shell appears to have been removed. I believe the image "Divider device 1" explains this as they have attached something to it. The test might have been a test performed in conjunction with the actual missile arming, firing and fuzing system, with this attachment simulating contact fuzing.
The only problem with this theory is that SRAM-II/SRAM-T were cancelled a this point. But there is precedent for them finishing up development of a system if it was almost finished at cancellation (W86 warhead for example). They may also have gone ahead with the test if Los Alamos saw the device as being important for future weapons. I've heard that the W91 was their "New Mexico 1" device which they proposed for the Reliable Replacement Warhead program.
Also, for those wondering why the aeroshell is included in the test, some devices were apparently sensitive to changes in mass near the radiation case as this extra material provided a tamping effect. I know that the W76 is one design sensitive to this.