A quick look through the Blackburn Roc pages in FAA Aircraft 1939-1945 suggests that whilst lots of Rocs went to lots of MUs, only a (relative) few went to 20MU, and those that did (L3101-L3109) were at best there up to Feb 1940, ie before the arrangement was finalised with Finland. Date range is (for example) L3101 20MU 8-18/10/39 then to RNDA (Royal Navy Deposit Account, apparently for FAA aircraft held in storage by the RAF) 20MU until issued to 806NAS 8/2/40. Other examples are L3104 20MU 8/10/39 then 759NAS 11/11/39, or L3108 and L3109 which jointly were 23/10/39 then 2AACU 1/11/39. The only possibility is L3107 which was delivered 20MU 18/10/39 then PD (Papers Deposited, essentially where the SOC date was unknown but the aircraft was no longer a viable concern) 5/9/40, with no other details.
This is the case for all Rocs as detailed in the book: the Roc was delivered to its MU then within a few months it was delivered to its operating unit or. in a few cases, a storage section at an FAA station before onward dispatch to its unit. If the details are correct in the book then maybe it was another maintenance unit that was to supply Rocs from its stock, or maybe the Air Enthusiast writer was right and the records did not mention each Roc being dispatched to 20MU.

Now, this requires that FAA Aircraft 1939-1945 is correct despite the fact that there is no mention made of this event that involved FAA aircraft (my copy in dated 1995, so it might be that any update does mention the Finnish affair); I have no idea when this event surfaced, although I have a copy of FAA In Camera 1912-1996 by Roger Hayward which has this picture in it (FAAM ref:ROC/39), so it has been around since at least 1996.

I have no idea how to access any ORBs, unfortunately, but would be interested if anyone can give them a glance.

Found this What If… whilst researching for this thread:
http://www.alternatehistory.com/discussion/showthread.php?t=185434&page=29

Without realising what that post was about it initially caused me great confusion – I knew the Finns did not get any Rocs, but the idea that they might adapt and improve them rather than curse their rotten luck is interesting. Finland was not the luckiest of nations regarding some of their aviation acquisitions, but despite the fact that everyone else used the Brewster Buffalo more or less because there was nothing else available they used it with much success, so the idea of them using it against the Soviets is food for thought.