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German eBay has a seller selling a tonearm from an Electrola 106-U portable gramophone. I'm thinking of picking it up to see if I can use an HMV 5B soundbox on my HMV 101 without having to adapt the isolator ring, since the tonearm is the same geometrically but has the larger 5 series soundbox fitting - to which a No 4 with the larger isolator ring would have been attached.

Here's my question: does anyone know if the two parts are interchangeable? The 106 tonearm seems to have a mounting flange on the base, through which the bolts pass, but the HMV 101 does not have any mounting flange at all above the motorboard. You can see three bolts, but they area short distance away from the visible base.

Michael, there is an easier way. If you happen to have a no5a sbx with the back rubber collar removed, you can fit into the sbx back a Meltrope red rubber connector. It fits tightly only by hand. And there are connectors with the same outer size but the inner size is different for the narrow tonearm of 101 or for the larger bore 102 tonearm. With these rubber connectors, you can install a 4 or a 5a in your 101 in a secoond. This way they are perfectly interchangeable.
I do use these in my daily soundboxes, and it is perfect. Besides that, as long as you've discarded the old original rubber with the brass insert and the locking pin, the nude red rubber connector allows you to adjust the angle of the needle on the record, which gives much more room for adjusting a perfect tracking with different lengths of needles, etc.

No, the two are not interchangeable, I've tried it! The 101 arm has a much deeper base, so mounting an Electrola 106 arm would leave a large gap between the underside of the base and the internal horn.

Barry