Something I have never seen before

possibly a Manx item

Due to cramped space, not accurate. The -68 650SS has Commando type rubber gaiters so hard to tell exactly. The -59 Manx easier to measure. Slider length seems same. Less than 1/2" difference for sure. Both have equal length left and right.

Edit: Also checked on the -58 99. Same length as the other bikes. 274mm from spindle centre to top of slider. Plus/minus 3mm.

See Attachment

By mudguard brace, do you mean bridge piece or stays. Inch and 3/8 is a considerable amount of difference.

Has someone fitted a (modified ?) Long Roadholder slider ? A photo would really help.

See attachment for a few variations. I am informed that between 1947 and 1977 there were no less than 17 different versions of the Roadholder Slider.

In the attachment A is immediately post-war. B & C are possibly Rigid and Plunger frames, D is Long Roadholder and E for the Short version. This needs confirming.

The other photo (Forks various versions) includes a few Manx and more earlier variations.
 

My guess is that your sliders are not different lengths.

The one which appears shorter (which I would bet money on being the pinch-bolt side) is, by the sound of it, a Long Roadholder one, which are machined down by 1 3/8" from the top to accept a tubular chrome shroud which conceals and protects the oil seals and (external) springs. the seal holder is a short one which recesses into the slider, like a Commando. Are there two very small self-tappers holding it on?

The mudguard lugs are further down as a result, but the sliders themselves are the same length and the seals, dampers etc are all the same or interchangeable.

Most likely in the 60s somebody broke the slider and replaced it with a cheap old one from a scrapyard, rather than pay for a new one, one of my bikes was like that when I got it, and the forks worked perfectly like that, but it does indeed look odd.