Identifying optical pickups/ laser units

I have two CD players (Marantz CD4000 and Arcam alpha 8) with dodgy reading/playing behaviour, and the logical step is to replace the optical pick-up, which on these budget machines isn't cost-effective unless you do it yourself. The part number isn't at all obvious, on these units anyway, although thanks to the kind help of forum members like yourself I was able to identify them.

Also, knowing which units are obsolete is useful when considering buying a used player, which is probably too late if my Arcam CD23 decides to go the same way. Any insight into that one?