The cost of a DR upgrade and service is £1500, which is kind of irrelevant, as you can’t have an Olive one upgraded, but for £1500 plus the sale of your 250, you could just about buy a used 250DR.
OMG, I seem to have turned into an accountant, but you get my drift…
Apologies for jumping in, but I had a related question about a second hand 250 CB that is for sale where I live, apparently in excellent condition. Putting aside service history, which I will check, performance wise would be worth grabbing. I could use it with a Nova or Uniti2, also have a SN2. Is the 250 CB any better than the more recent power amp stages in any of those 3?
Units that were born a DR have the ident integral to the rear label/panel (as in Richard’s picture), units that have been upgraded should have the metallic sticker (as in your pictures). The position of the sticker on upgrades may vary depending on the individual who carried out the work.
Regards
Neil.
Edit: I failed to answer your original question - the amplifier DR upgrade option was offered very soon after the DRs went into production, a month or two after at a guess. So around 2015 sometime.