I have been trying to find out who made the Systemdek arm and I see it is very close to the AR arm. I was told a while ago it was Jelco but I couldn't find any info confirming that
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.
Did you use the same hole? Or did it require a new arm board (overhang and size of the hole etc)
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.