I found this quote on QRZ-
"You can also get on 60 meters by performing another mod that throws all frequencies (going off memory here) 3MHz lower than what is displayed. (Mine doesn't even have the WARC bands enabled.)"
I found your conversion-
Put one switch positive (+5v)/negative(ground)of PIN5 of I.C 76LS163N wich is controlling PLL frequency
In the TS 120 the IC is located in PLL unit X50-1490-00
In the TS 130 the IC is on PLL X50-1700-00
On the schematic diagram 74LS163N is Q16 on TS120 and Q23 in the TS 130
Near IC you will find connectors numered as J15 both in TS130 and TS120. You can see the line numer as A1(1), B1(2), C1(3), D1(4), 5V (positive), BL6 on these conectors.
The C1(3) is lined into PIN5 of 76LS163N I.C, just add the switch between c(1) and 5V(positive) (5), Then you can swith positive/negative of C1(3).
With this mod all Frequency is exactly 2 Mhz converted, but your display don´t change, so you must read 28Mhz as 26 and 29Mhz as 27Mhz when the switch is ON.
So apparently you have a switch.