Resigning itself to the fact that it would fall to MAI Basic Four Inc’s $20-a-share bid unless it could find a more attractive home, Prime Computer Inc late Tuesday put itself up for sale, saying that it would entertain offers that exceeded MAI’s $20 a share. The market is not confident that it will be able to find a buyer, and the share price opened at just $20 yesterday morning, having put $1.875 to $19.875 on the announcement. Prime has commissioned First Boston Corp and Smith Barney Harris Upham & Co to solicit bids for the company, but it is not easy to see a serious bid coming from a US company at over $20. Prime tried hard to persuade Ford Motor Corp to don white armour to no avail, and Lockheed Corp is a seller, not a buyer, of CAD/CAM interests. General Electric and Rockwell International are long shots, but it seems more likely that any bid will come from abroad. Prime is unattractively visible for a Japanese company in the present climate, but Siemens AG, which has expressed interest in principle, is just one of a string of West German and French and British companies that might see Prime as a serious entree into the US.