1938 vs 1939 oil tank

From what I have heard and seen there are two types of the late "embossed" style knuckle oil tank (embossed with oil nipples) as used on the 38 & 39 big twins. The difference lies in the underside. One style has a seamless bottom and the other a rolled edge. Experts tell me that 1938 uses the seamless type and that 1939 uses the rolled edge style. I have also heard that late 1938 could have used the rolled edge style. My question is simple: does anyone know when the change actually occured? Is there any factory document relating the change? Does anyone know of any unrestored bikes that could be looked at? Any help would be appreciated. Thanks,
Eric

The early 1938 tank has a bottom like 1937. Later 1938 uses a tank like 1939. No documentation just lots of observations.

Jerry

Thanks Jerry,
This confirms what I thought. I am working on a bike in the 2700 serial range so I'll go with the later style. This bike actually came with a 1937 oil tank and is, as far as I could tell, all original. It's an export bike (France) and I heard that the factory got rid of the obsolete parts on the export models so it could actually be "factory". I am putting it back to "normal". In a way that's a mistake I guess.
Eric