CVI solid Etalon part # ET-24.4-0.30-UV, 0.3mm thick, Fused silica, 890nm, 20%R

Free spectral range at 600nm (cm-1)  = 11.2  (according to my catalog).

Looks unused and has writing on box " LLNL, Indigo-S etalon" 

I don't know of any laser called Indigo associated with LLNL.