Running st_crs("+proj=lcc +lat_1=75 +lat_2=50 +ellps=WGS84")
gives me:
CONVERSION["unknown",
METHOD["Lambert Conic Conformal (2SP)",
ID["EPSG",9802]],
PARAMETER["Latitude of false origin",0,
ANGLEUNIT["degree",0.0174532925199433],
ID["EPSG",8821]],
PARAMETER["Longitude of false origin",0,
ANGLEUNIT["degree",0.0174532925199433],
ID["EPSG",8822]],
PARAMETER["Latitude of 1st standard parallel",75,
ANGLEUNIT["degree",0.0174532925199433],
ID["EPSG",8823]],
PARAMETER["Latitude of 2nd standard parallel",50,
ANGLEUNIT["degree",0.0174532925199433],
ID["EPSG",8824]],
PARAMETER["Easting at false origin",0,
LENGTHUNIT["metre",1],
ID["EPSG",8826]],
PARAMETER["Northing at false origin",0,
LENGTHUNIT["metre",1],
ID["EPSG",8827]]],
Which tells me your four missing parameter defaults are all zero.
So these two should be equivalent:
c1 = "+proj=lcc +lat_1=75 +lat_2=50 +ellps=WGS84"
c2 = "+proj=lcc +lat_1=75 +lat_2=50 +lat_0=0 +lon_0=0 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs"
As a quick test lets project some lat-long coords to both and see:
pts = st_as_sf(data.frame(x=runif(10,-180,180), y= runif(10,-90,90)), coords=1:2, crs=4326)
p1 = st_transform(pts, c1)
p2 = st_transform(pts, c2)
st_coordinates(p1) == st_coordinates(p2)
gives:
X Y
1 TRUE TRUE
2 TRUE TRUE
3 TRUE TRUE
4 TRUE TRUE
5 TRUE TRUE
6 TRUE TRUE
7 TRUE TRUE
8 TRUE TRUE
9 TRUE TRUE
10 TRUE TRUE
which convinces me.