I added into my earlier comment, last week:

HP-LaserJet-6P-Printer-Postscript-Level-2-Module-1MB-Memory-C3987-60001

because there was on e8ay such an item, for about £10 (Euros 11), but I believe that one has already sold or ended, but there is another one, but it's in USA for about £30 (Euros 33) - item 332556397073

I thought you might have gone looking for such an item, armed with the information I passed, but it seem you've either missed the bargain one that was previously listed, or will have to wait for it to be re-listed or choose the one I've mentioned above - if you still want to acquire one of these PostScript SIMMs for your model of HP LaserJet printer.

Of course it's up to you if you want to get the best out of the printer you have.