It is indeed a Jelco arm. As well as adorning the Systemdek the same arm was seen under several other brand-names at the time. Mission comes to mind.

I replaced mine with a Rega RB300 after a few months with no regrets. It took the tt up into a higher category, especially when using mc cartridges. I more recently replaced the RB300 with a Michell TecnoArm (a heavily modified RB250), which is just the bees knees.

A new arm-board was required. For an RB 250/300/Michell a distance of 222mm between the record spindle and the hole center is necessary for successful cartridge alignment.