You said the apartment was inherited, but it's not clear whether you inherited it directly from your great-grandfather, or there were intermediate transfers of ownership. How did you become the owner of the apartment? If you inherited it, your basis is the fair market value on the date of death of the person you inherited it from. That should be easier to determine than the price that your great-grandfather paid in the 1940s, and will be considerably higher than the original purchase price. You can probably get a retroactive appraisal from a professional appraiser.
