[Old Henry]

Don't know if this would apply to your modified '36 but on my '47 I always get a little spark hooking up the battery because the electric self winding clock has unwound and the contacts are closed to energize the electro-magnet to rewind it. When I hook up the battery a split second connection that immediately opens shows as a spark. Could be as simple as that.

As others have said, you really need to check the magnitude of what you are thinking is a continuous drain with a meter. May be just something like the clock or a condenser charging that makes the spark but is not really draining. Even just a test light between the disconnected battery clamp and the post would show that without a meter. If it just flashes on then off - no drain. If it stays on bright - it's a definite drain.

If you're not sure about your battery, wouldn't hurt to have it checked. They do wear out.