A sliding SME base

 Here are photos, kindly sent by Chris, of his RP1 on his Thorens.
It is using an SME adapter, ostensibly for Ortofon arms, which comes from Hong Kong.
The ebay link is here. I have also seen them advertised in clear anodising (silver finish). Another black option is here

It looks good and isn't too expensive. As a finishing touch, an option is to paint the silver knob black, or get a gold plated one.

Adjustable Headshell

Some more information on the last headshell variant: this is a gold plated, high mass version with a slot for adjusting effective length as well as offset.

The standard version omitted the slot:

And one with a sapphire:

The earlier version, by comparison. is not quite as elegant:

The headshell is in two parts with the cartridge being fixed to the lower part as normal, then attached to the upper part using a screw which is positioned above the stylus.

Headshells with slots allow for adjustment of overhang and offset, but inevitably when adjusting one, the other is affected. Separating the two adjustments is a good idea. However, most headshells which pivot to allow for adjusting offset angle do so from a point between the cartridge mounting screws. I have never understood the reason for that because when adjusting offset the stylus swings in an arc so that the effective length is changed, and so the advantage is lost.

When making this headshell, I positioned the pivot above the stylus so that there is little or no variation in effective length as the offset is changed. In this way overhang, which is adjusted at the base, is a separate adjustment, and so cartridge set up can be easier, more accurate, and less frustrating, as once overhang is set, the offset can be adjusted without the stylus moving back and forth.

How I would design an arm now

Well, rather than give the game away completely,  and since I've commented recently on another arm design,  here's what I wouldn't do.

If I used ball races, which I might do, I wouldn't align those for vertical movement in any way other than to have the cartridge move vertically, in the plane of the cantilever, and the counterweight parallel to that.

The axis of the bearing for vertical movement would not intersect the arm tube axis, or be above the stylus.
The axis of the bearing for horizontal movment would not intersect either of the above axes.
The counterweight would not be on the same axis as the arm tube.
There would be no arm "tube".
I wouldn't have a fixed geometry, as all parameters would be adjustable.
I wouldn't omit anti-skate.
The arm wouldn't be symmetrical
I would avoid manual adjustments.
I would not  have just a 9" version.

As far as the old RP1 is concerned, the only things easily changed without altering it too much would be:
1. to provide a sliding base with better adjustment of arm pillar height (an adaptor for Ortofon arms which is now available on ebay will do the job)
2. redesign the headshell to provide variable offset (which I did on the last few arms)
3. provide a secondary counterweight for adjusting downforce
4. maybe a secondary counterweight to improve lateral balance