Are you certain about the 74LS387 part number? I can't even find a data sheet for that.
the MC10231 and MC10102 are both ECL parts and seem to be available (-ish) on the auction site. I can easily find data sheets for them, even listings on Mouser (that say they are obsolete and not available).
The only reference I can find for the 74x387 is on Wikipedia, which says it is a 1024-bit (256x4) PROM. It does not indicate that there was an LS version, only an S version. Do you have the PROM contents for the chip you are replacing, or a way to program the blank PROM if you had one?
-- Jeff Dutky
