I have a 2 stroke Mercury outboard 50 horse with no serial number.. where else can I look for a number to order a correct lower unit

Question edited for clarity (correct order).

https://www.manualslib.com/products/Mercury-55-Seapro-5548103.html

More here
https://www.google.com/search?q=Mercury+55+HP+outboard+motor+manual

serial number guide

https://www.marineengine.com/mfr/mercury_models.html?ref=tfrecipes

..

need the year of manufacture as some 2 stroke units have oil injection that automatically mixes the oil in the correct ratio for engine rpms otherwise it is 50:1 ( 50 parts fuel to 1 part oil)
oil requirements --certified tc-w3 2 stroke outboard oil ---not ordinary engine oil
reference
google --type in mercury outboard oil to fuel ratio

Model # shows it's a 1973 year, 50 horse power. 2 stroke engine with 50 to 1 fuel mix.

the serial number begins 2487558 and continues past 2491265 and as the beginning serial number denotes the year , your engine is 1969 model
it shows as a 2 stroke with auto oil injection
check the heads
if the plugs are in the flat of the head and there are no rocker covers it is 2 stroke
talk with a mercury /mariner service center

Go to boats.net - Mercury parts - search under hp/serial range - select 20hp 2 cylinder - it will be under the first selection 0A730007 and up.

There is no data included in the serial number that gives info of what year the engine was made. You can, however look up the number here, and that will tell you the year of manufactor.

http://www.iboats.com/basics/serial_number_year.html#mercury

pinched rom another website:
Every Mercury outboard has a serial number label located on the transom bracket area of the engine. The latest serial number labels display a 2-digit number in a box at the lower right portion of the label. These digits coincide with the last two digits of the year in which the outboard was manufactured. For example, if an outboard was manufactured in 2010, the number "10" would appear in the box at the lower right portion of the label.