Let's do this...
Here is a merge of the 41a set of Convoys, Merchants, TaskForce & Troopship, on the date of 28 Feb, 1941. Notice the traffic level. This is the last date of the 41a layer, when all RGG will cease to SPAWN - not operate, but will no longer spawn.
I showed an image above of the 42a merged layers at the beginning of April 1942. Refer to that above in a previous post. It is nearly a fully spawned layer by this time. Now, here is the 41a merged layers on 15 March 1942, which is the RGG that had spawned on or prior to 28 Feb 1942, still traveling to destination:
Here is the 42a layer early on 10 March, 1942 (almost fully populated):
If I put those together, through the magic and wonder of PhotoShopLE, I get this:
Now, I wasn't very good at my screen grabs, but it gives you the idea of the point I am making:
There is no such thing as a "gap" between layers. As the one layer fades out, the other is fading in. You will find lower spawning rates, but the previous layer traffic is still traveling. The 41a layer doesn't quit completely until mid 1942, but the 42a layer is mostly fully functional by April 1. If anything, you will find MORE traffic at the "transition" time frames. This applies to TMO, Stock, RFB, RSRDC, DW, OM, WDAD, etc.
PS: I very much detest the "New" Windows and its displaying of advertisements on my OS "Notifications" area, and I can't close them without opening the advertiser's web site. Who came up with this idea?... sheesh! How do I block this crap, which used to be called "spam"??