So it's time to change the impeller, 1st time with this motor (D1-20), but I've done Yanmars before. The PO left a bag of spares, including 2 new impeller kits.

My problem is not knowing which one to use(or even if its the correct part), and i won't start the job until i know the replacement is correct.
The impellers are physically identical in size and dimensions, but differ in the shaft - one is pressed in and smooth, the other threaded and screwed in. I'm assuming the slightly different part numbers account for this.
I have the correct Owners Manual (pretty pics only), the correct Workshop Manual (detail, but no numbers), and an exploded view of the pump (below), but the part no. shown(part 18) bears no resemblance at all to the numbers on the boxes.

Is it a generic part? or is it engine specific, and where would i find the right numbers? (If it is generic, which one should i use?)
If somebody could point me in the right direction I'd really appreciate it.
Thanks very much.