No idea what the 'SL' is for. I also have an Ariston RD110SL
These decks were designed when the Ford Cortina was around and I have always supposed that in common with cars of the era (models had L, GL, SL, GXL etc as insignia) the SL was to stand for 'Super lux' or some such.
Even some cameras of the era joined in the madness, you can even get a 'GT' Yashica!, only available in 'racing black'.
Back to reality, manicmanta, do you use the standard Ariston rubber mat. the ones that I have are over 4mm thick, which does not leave much, plain unthreaded spindle for the record to centre on.