The partnumber of this piece is unknown to me.
I have a lot of part manuals but none of them show this X plate like in this early exploded view. (Note: See the two piece cilinderhead engine tin)
Top x cover plate is used by VW on the 2-piece crankcase.
Bottom x cover plate is used by Porsche on the 3-piece crankcase from 1954.
This is the cover plate used on the 25&30PK crankcases.
The online Porsche PET 356 pre A
will guide you towards Porsche part number 616 106 083 00 which was renumbered from 539 06 524.
As you can see below this part will not fit the two piece crankcase.
Used up to up to engine number 7 336 419 (11/1962)
Is called Stale Air.
From engine No. 20-1 120 615 (05/1955)
1963-1970 , made in germany.