The cylindrical keycaps I ordered arrived in today
All my default Maltron keys are of the spherical type, most visually similar to cross-sections of the DSA family of keycaps, though at a different physical size.
sampling two single maltron keycaps gives measurements of:
81.5mm (0.32"), 8.06mm (0.317") tall
18.09mm (0.712"), 18.04mm (0.710") long square base sides
the shallowest dip for each key is within the range of 0.8mm to 0.9mm (0.31" to 0.35") but it isn't easy for me to measure this consistently
The first of these keycaps is from a maltron keyboard more than a decade old, the second keycap is from a set bought direct from maltron around a year ago.
thankyou for all the information, time for me to try out some keys and think about what happens next.