Received some additional photos and information from Jerry Beasley and Jerry Gilmartin. Both of whom know far more about this subject than me.
From Jerry Beasley...
Great questions and some good follow up on tracers and the make up of individual clips. While I'm a PT modeler I also got bit by the ammo bug and have been a serious collector of .30 cal, .45 cal, .50 cal, 20mm, 37 mm and 40mm ammo for the past 10 years or so. It started as all the ammo carried on a PT boat but my wife drew the line at depth charges and torpedoes - I am however always on the lookout for a 5" rocket if anyone (hint: Jerry G) knows where one might ever be available. I'm also involved with restoration of military aircraft which carries over in to the full restoration on the ammo.
I have never posted a photo on the forum so Dave Waples has agreed to post them for me.
In my experience I have never seen a mixed clip of 40mm ammo used in an operational weapon, whether used in ground vehicles such as the M-41 Duster, shipboard usage or airborne platforms such as the Specter gun ships. As reference each of the ammo boxes or cans that I have in my collection or referenced in photo sources have the ammo type and nomenclature marked on the outside of the container and in my experience are limited to a single ammo type.
First I included a chart of the 40mm rounds used in WWII by the US Navy - this was put together by a close friend and fellow collector and he gave me permission to publish in the forum and for it to be used by any of the forum members in you would like - I am going to do a new one that will show all the proper nomenclature that should be painted on each projectile - the type of round is on one side and the lot information is stenciled on the opposite side.
As to Murphy's Laws of Combat that "tracers work both ways" - very true, but you will see reference to a "dark" or "dim" tracer as well as a delayed ignition tracer. While a lot has been written I think in the end they are really all the same animal. The problem especially with large caliber tracers is that when they ignite as fired they can destroy the gun crews night vision, and they draw a straight line from the target right back to the barrel of the gun. The dark or delayed ignition tracer has a very slow burning ignition train that allows the projectile to travel several hundred yards from the barrel before igniting the tracer element. The crew maintains their night vision and the individuals on the receiving of the incoming rounds can only guess at where the shooters are located.
As to the sequencing in the 37mm cannon, I suppose a lot would depend on the individual target type being engaged. There are a number of good pictures of sequencing in the airborne version of the 37mm, but I was able to find one good photo from what appears to be an operational WWII PT boat. It is in black and white but with the color differentiation as well as the cartridge length it is clearly sequenced HE/AP/AP/AP/HE/AP/AP/AP.
40mm Identification Chart - USN
Examples of 40mm projectiles
Examples of 37mm on USS Alaska
PT-305 40mm restoration. Nice work on the 40mm ammo
37mm Load Out