1) mk1 ( 5 lines floating ) : R and early L series ( inverted 6 )
2) mk2 ( 4 lines ) : L series up to around 500.000 ( inverted 6 )
3) mk3 : ( 5 lines ) : from L 500.000 up to around S 500/600.000 ( inv.6 )
4) mk4 : ( 5 lines ) : same range as mk3 ( inverted 6 )
5) mk5 and mk 6 : from around middle/high S up to A series ( standard 6 )
a big thank to the italian collector PINO who has done the deepest studies ever made about the 16520 and the present tablet.
