I guess I should amend the;
#mlat_cmd=/usr/bin/python3.5 /usr/bin/mlat-client
shown in the screenshot above to read;
mlat_cmd=/usr/bin/python3.7 /usr/bin/mlat-client --results beast,connect,127.0.0.1:30104
as per your post above?
The command "mlat_cmd=/usr/bin/python3.5 /usr/bin/mlat-client" with correct python version 3.7 is built into rbfeeder. That is why by default in the file /etc/rbfeeder.ini it is commented out (starts with #) and is therefore ineffective. In nano any line starting with # appears blue to highlight at a glance that it is commented out and is ineffective.
When rbfeeder is started, the builtin correct command executes automatically, provided you have removed # from start of line "#autostart_mlat=true" (in nano this line will change its color from blue to white when # is removed)
Leave the line "#mlat_cmd=/usr/bin/python3.5 /usr/bin/mlat-client" as is. Since it is commented-out, in nano it will show blue color. Let it stay blue colored.
In case you want to over-rule builtin command and use a custom command with additional parameter such as "--results beast,connect,127.0.0.1:30104" (for feeding results to dump1090 fa or mutability), you have to use "mlat_cmd=/usr/bin/python3.7 /usr/bin/mlat-client --results beast,connect,127.0.0.1:30104". Since you want it to execute, there should be no # at its start (the line will show white color in nano).
In case the custom command gives issues, simply make it ineffective by placing a # at it's start. Upon placing a # at start of line, it's color will turn blue in nano confirming that it is commented-out and is ineffective.