Series 3 splined fibrous washer

I am new to this and I may not get the terminology right, so be patient. I had to replace the oil seal due to oil seeping into the transmission brake. Managed that ok but in the process I have to replace a fibrous washer/seal (call it what you like 'cause I can't find it in my Haynes exploded diagram) which fits around the prop driving shaft and into a splined aperture. I am attaching photos because it will be instantly recognizable to those totally confused by my description. Also a thumbnail of the old and new fibre insert.
Problem is I have ruined one as I can't seem to compress it to the correct size and shape to fit the splined aperture. The original seems to be of a more spongy material (even that would be a challenge) whereas the new does not want to be compressed. Is there a technique or a trick to this because my landy has been standing with its nose in the air for a couple of years now, waiting for me to get round to it!! Having had my three score and ten years, going into contortions under the landy is bad enough but getting skinned knuckles etc and still not succeeding kind of frustrating. Advice [of a friendly nature] would be greatly appreciated.
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I've never done this job so I'm only guessing but looking at your pic, the spline look flush with the surface of the other round thing with the studs in. But the spongy washer has cut outs for the splines, so I would have thought the splines should be protruding. Is the round thing with the studs too far forward. Sorry for being so technical.

Col

Thanks Col, no it isn't flush. There is plenty of room to insert the fibre washer/seal .....if only I could.

The felt washer you have is not the correct one. Should be felt and rubber.
Should be part number RTC3515
What you have looks like 622042 which is for gearbox output shafts

Remember not to over tighten the castellated nut and squash the felt washer, just tighten it until its hand tight and you can fit the split pin in the nut