Hi,
as far as I understand this, it does not matter how many hours you have flown the variant, but it matters that you have done the difference training at an approved ATO and WHEN you have flown the 605 for the last time.
IMO, you must have flown the variant within the past 24 months.
Also FCL SAYS in FCL.710 Class and type ratings — variants
(b) If the variant has not been flown within a period of 2 years following the differences raining, further differences training or a proficiency check in that variant shall be required to maintain the privileges, except for types or variants within the single-engine piston and TMG class ratings.
So even if you had done a difference training for the 605, but haven't flown it for 2 years, you must either do one more difference training, or a PC in the 605 variant (the 604 does not qualify to keep the 605 privilleges going...) An OPC also is useless as it is not a FCL event, but an operator specific thing with no official syllabus. I know many operators use the same syllabus, but officially there is none.
Happy landings