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The front bearing retainer, that acts as the pilot for the throw out bearing, has no internal seal on the 49 through early 51 transmissions. This acts as a vent for the front gear section and possibly the overdrive section as well. There is a baffle in the front bearing stack up there to prevent leakage on the main input shaft.
You will need to identify where the leak is coming from. Those old rear seals werent much more than a big felt washer where the slip yoke goes. If the car sets on an incline, you can get oil seeping into areas where it doesn't normally pool and that can cause some leak out when the car sets for a while. If it continues to leak, it might warrant further inspection for bad gaskets or cracks in the case.