The 300SE fintail sedan came in two versions. Chassis type 112.014 (the 300SE) has a wheelbase of 2750 mm while chassis type 112.015 (the 300SE long, never 300SEL) has a wheelbase of 2850 mm. This is not really the same long wheelbase as a 600 where an entire extra door or panel is inserted. Long in this case is analogous to the 280SE vs. 280SEL sedan or the 450SE vs. 450SEL sedan. Total production of the 300SE long was 1546 so this makes it extremely rare. Especially when you consider that most of them have been lost to rust and the crusher.
On the subject of long wheelbase fintails, it is appropriate to mention that Mercedes also produced an extended version of the 200D (chassis type 110.110) and the 230 (chassis type 110.011). Both of these have a wheelbase of 3350 mm and do have an additional panel like the 600 LWB, They were available under option code 846 with a seating capacity of 7 or 8 persons and option code 847 with a seating capacity of 5 or 6 persions. However, whereas the 300SE long was the most luxurious fintail produced, the 200D / 230 extended wheelbase were utilitarian vehicles often used by hotels as airport limosines and the like. You can see one of these in the 1969 film by Alfred Hitchcock entitled "Topaz". Finally, to avoid the question before it is mentioned, these extended wheelbase vehicles were made by Daimler-Benz; they are not special coachwork produced by Binz or Meisen who produced ambulances/hearses/station wagons or IMA in Belgium who produced the Universal.