The Mk series were developed on the basis of the French GA MMN bombs - Gros Andreau bombs were the liquid Anilite explosive was replaced by MMN.
Such bombs became common in 1918 in the French Air service, as they avoided the many problems of the Anilite and its toxic effects for the crews.
The Gros Andreau were available in 10kg (120mm diameter), 25kg (155mm diameter) and 50kg (200mm diameter)
According to the Operational Records of French Bombing Squadrons, The 10kg (120mm) diameter had been the most commonly used in the war and after that the 50kg (200mm)
The GAMMN bombs could be fused in case of horizontal suspension with a Michelin nose fuse installed instead of the Gros Key (used to puncture the separation between anilite and hydrocarbure inside the bomb at dropping time), or with a tail fuse Michelin No2, the Gros key being replaced by a vertical suspension key.
In 1918 these bombs became respectively
10kg GAMMN 25kg GAMMN and 50kg GAMMN
However only the 50kg GAMMN remained standard issue after 1918.
The bomb rack on the picture is of a type designed for a "50lb and over" order of bombs. The 25lbs typically used a multiple rack, and in any case was relegated after June 1918 strictly to exercise (and remained in such role till the end of the 1920s)
So it is most probably a 50lb or 100lb bomb.
So we may have here a very early Mk series bomb, still very similar to the GAMMN bomb, though the nose fuse looks definitively American
However, it's most probably a 100lb MkI MI as this bombs was, in the early 1930s, equipped with a nose fuse MkVII MI, while the 25bs and 50lbs were simply discontinued and eliminated from the stocks.
This is a drawing of such a bomb from a document by Henry Belot
The 50lb had a length of 930mm and a diameter of 155mm (like its French model)
The 100lb had a length of 1257mm and a diameter of 200mm (here too like its French model)
BTW The early Mk series suffered from the same defects of the original GAMMN bombs - too thin metallic envelop, with too much soldering between parts - that led to the French abandoning them for better bombs.