Some kind of US Military Standard Electronic Module, see http://www.tpub.com/content/MIL-SPEC/MIL-M/ (the marking M28787 is the MIL-M number) Referred to here: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.129.4343&rep=r… No idea what they are/were used in. If you Google for M28787 there are dozens of surplus supply outfits selling these. /Jonas