it does not have to be that old 5.2 loader
when you have a 3615, updated to 6.2.2 and want to downgrade its also possible to use loader 1.02b for 3617 and on migrate select the 2nd option to keep only data (loose settings), install "DSM_DS3617xs_15266" and from there you can decide to migrate to 3615 dsm 6.1 or updgrade to 6.2.(0) 3615 or 3617
but that 6.2.2. downgrade is history in a few days
i already have a working extra.lzma for 3615 DSM 6.2.2 that contains all drivers the version for loader 1.02b had, it only needs some documentation and testing