i always thought i have an L block in my '91 240. the timing cover sticker said L (before the adhesive gave and the fan blew it off to oblivion). today i was lying under the car and saw "B230 K" cast on the block. didn't K blocks get phased out by '89?

The previous poster is correct, L blocks started in 1993.

If you saw L on the engine, the upper timing belt cover could have been swapped. The L engines only began in 1993 (last year of the 240s) but continued several more years in the 4-cyl. 900 series cars.

The actual L (in the context of what you're asking about) on the engine label would be a slightly smaller font-size letter in the upper row (sandwiched between two rows of bar code), following a seven-digit engine code number in a larger font-size. In the second row (under the second bar code), it says B230F and then some other engine number (e.g., like this: NR. 577).