1670cc engines

[GeoffJohnson]

September VL's article on Stainless Steven's Aprilia states "The Department of Transport V5 document......tells us that the car was still green but the engine number was 8314 with a cubic capacity of 1670cc!". A while ago I came across a picture on Flickr, taken by Robert Knight, of an Aprilia at Prescott in 2011 with the comment that the engine size appears to be 1670cc. The car, FLE859, is shown as belonging to Mr A W Rippon in the 2011 members list. I am interested because my V5 also shows 1670cc. Was somebody modifying engines to this size? Or is it more likely that the DVLC "guesstimated" the conversion from horsepower to cc when changing from the old log books?
Geoff Johnson