Pete,
From what I can work out Squid and Limbo used the same depth charge. I think the difference in nose profile could be explained through difference in model, the item in photo three is marked as a Mk 6 and has a 1980 date of manufacture.
TimG
I know that this is a very old post but I am the "New boy" here and still trawling through the old stuff. The main difference between the Squid and Limbo (Mortar Mark 10) was that the depth settings for the fuze were passed to the fuze by a cable on the Squid and via a needle and contact ring on the Limbo.
The squid cable was coiled up beneath a metal lid on the nose and was connected to a junction box on the outside of the barrel. The cable was cut off as the bomb was fired, a trailing length flew with the bomb. If bakelite muzzle covers were fitted to keep the weather out of the loaded barrels then the cable was passed through a rubber sealing grommet in the centre of the cap before connecting to the JB.
The Limbo bomb had an insulated metal ring around it below the curvature of the nose. This ring was connected electrically to the fuze and had a rubber sealing cover on the outside. The spring loaded needle pierced the rubber to make contact with the ring. A solenoid withdrew the needle as part of the firing sequence to prevent breakage.
There were practice light bombs and also heavy bombs. The practice "lights" (no fuze and flat nosed) were fired from the Mortar mark 10 with a light charge to give the same range as a heavy bomb with a full charge. Old squid cartridges were used, with a steel chamber adaptor, to shoot the light bombs. It was intended that these should float and be recovered for re-use. There was a "weep" hole in the nose, covered with a thin glass disc, which allowed the bomb to slowly fill with oggin and sink.
Very occasional errors were made with the Mortar mark 10 by firing a heavy (Live or practice) bomb with a light charge, one bomb landed on the forecastle deck and penetrated through a bunk in a messdeck. Conversely I watched six light bombs fired with heavy charges, goodness knows where they went, probably still going up!
gravelbelly