Richard, I would agree with you. I thought it unlikely that it would be a "part number" due to its make-up, it is only a piece of foam rubber about 20mm wide x 25mm high x 50mm long. I'm not actually sure what the thing does but it is glued on with RTV.
I am going to order a roll off the internet, which comes as 20mm wide x 25mm high x 2metre roll, costing around £7. I can't find anything of these dimensions on the websites of DIY stores here. I want to get a foam rubber, or neoprene rubber, as similar in make-up as the one currently on the door, just in case it performs some really important, but unknown, job !!!
I would imagine that the manager at LR, with responsibility for production, probably just rang a local motor factor and ordered 50 boxes of a foam rubber, self-adhesive, strip to go under the door card. So therefore no official part number.
Pierre3.