PART NO.  SPL.PAGE  DESCRIPTION                                                           
6K639     15.A.10   Screw with captive washer -reservoir to crankcase
6K639     16.A.9    Screw with captive washer -reservoir to crankcase (19) -(engine, 1600)
6K639     16.A.30   Screw with captive washer -reservoir to crankcase (19) -(engine,
    1622) -not available, use HZS0404 with LWZ404

6K639 is a special hex head screw, 1/4-28-UNF x 1/2" long, with an undercut below the head.
The washer is special 1/4" internal tooth washer with interference fit ID, so it has to be pressed on to be captive. When assembled, the internal fingers on the washer latch into the groove in the screw, so the washer will be captured but still free to turn. Originally provided as mated pair, the washer does not have a separate part number.

Notice, this is not an internal tooth lockwasher, as those teeth are not angled with sharp edges. The internal fingers are on a conical surface shape to slip down over the screw threads when it it pushed into place (without having to screw the washer onto the bolt).

As noted with the 16.A.30 SPL page for the 1622 engine, if the captive washer screws are not available, you can substitute a 1/4-28-UNF x 1/2 hex head screw and standard 1/4" lockwasher (in which case it will not be a captive washer).