Bulk Voucher Number as reference

I am try to copy voucher number to Reference number either for whole data or for selected vouchers

from the below code it only assigns for the first record from selected rows

Code:

[#Form : DayBook]
    Add    : Button    : After    : Fourth Blank Button    :Change details

[#form:DSP Add Save View Button]
        Add            : Button        : After                        : Fourth Blank Button    :Change Reference

[Button:Change Reference]
    Key: Ctrl + W
    Title:"Change Reference"
    Action:Call:Bulk change Ref
    Scope:Selected Lines
  
    [Collection : SelSrcCol]

        Data Source : Report : Selected Lines
        Fetch : MasterID , VoucherNumber, PartyLedgerName, Date, Reference, LedgerEntries.*

[Function:bulk change Ref]
    Variable:vNewRefnum:String
    Variable:Vch:String
    Variable:Ref:String
    Variable:countr:Number:1

    75 : WALK : SelSrcCol
        75b:Set:Ref:($$LocaleString:"ID :"+$$LocaleString:@@GSTCurntVchrMSTID)
        75c:Set:vNewRefnum:$VoucherNumber
        75d: Modify Object:(Voucher, ##Ref).Reference:##vNewRefnum
        75e:Increment:counter
    90 : END WALK