Based on what little I can see I'd guess it's a mid/late 70s 5ply. The dual spurs were a fairly common modification and standard equipment on some outfit bass drums.

What KO said about the double spurs is the way I remember it. The bass drums could be ordered that way or modified by the drummer or dealer to have the two sets of spurs.

24" & up usually had the double spurs... and I think the Avante kits also..

Cheers

John

Dual spurs on a 20" bass drum was very uncommon! Slingerland's dual spurs, model 727, first appeared on the BR kit. They showed up again on the Avante outfit, along with most other "rock" kits using bass drums of 22" and larger. It's possible this 20" was custom ordered or added after the fact. An easy way to tell is to remove the brackets on one side and see if the drilled holes look identical. The hole for the spur was drilled at an angle and can be difficult to achieve "factory-like" appearance. Remove those spurs and take pictures!

-Mark

1979 would be expected for serial 399xxx. Is there a cursive "Slingerland" ink stamp on the interior of the shells? That would also be expected. Marks suggestion for verifying a factory dual spur installation is a good one.

It looks all factory to me. Even the finish. The BR kit had them and I agree that it is something ordered.

What does the 109 ink on it mean ?

Yes, it looks factory to me, too...and probably a special order! Still, I'd really like to see the drill holes.

-Mark

Here are some additional pictures. The holes seem to be factory made, both spur holes have the same drilled angle. I have also attached pictures of the tom holders. The long tom holder is from a previous BR kit. Yes, 5-ply kit with M/P/M/P/M. Appears to be a jazz kit. This kit was bought new in Oslo, Norway around 1978/9 and only one owner since then.