TL;DR: Yes, in all the examples I've seen, but I've not seen some of the treaties binding as of WW2.

A handy source is Treaties and Documents: By topic But they appear to be only the current treaties. For example, it doesn't appear to have the original ten-article Geneva Convention of 1864, or the treaties of 1906 and 1929, just the four conventions of 1949.

Nitpicks on the query:

• There are four Geneva Conventions from 1949 alone
• Treaties concerning methods of warfare are almost all from The Hague

In the treaties I've looked at, the applicability is near the start.

It appears that usually all restrictions were off when a non-contracting party joined in, though it might be with respect to all or just when the non-contracting party joined. For example, Convention (II) with Respect to the Laws and Customs of War on Land and its annex: Regulations concerning the Laws and Customs of War on Land. The Hague, 29 July 1899, has "Art. 2. The provisions contained in the Regulations mentioned in Article I are only binding on the Contracting Powers, in case of war between two or more of them. These provisions shall cease to be binding from the time when, in a war between Contracting Powers, a non-Contracting Power joins one of the belligerents." (with provisions for new accessions and denunciations of existing ratification). The other 1899 treaties have the same language.

In contrast, Convention (I) for the Amelioration of the Condition of the Wounded and Sick in Armed Forces in the Field. Geneva, 12 August 1949 (past the time frame of your question; I'm using it as an example) has "In addition to the provisions which shall be implemented in peacetime, the present Convention shall apply to all cases of declared war or of any other armed conflict which may arise between two or more of the High Contracting Parties, even if the state of war is not recognized by one of them. The Convention shall also apply to all cases of partial or total occupation of the territory of a High Contracting Party, even if the said occupation meets with no armed resistance. Although one of the Powers in conflict may not be a party to the present Convention, the Powers who are parties thereto shall remain bound by it in their mutual relations. They shall furthermore be bound by the Convention in relation to the said Power, if the latter accepts and applies the provisions thereof."

Protocol for the Prohibition of the Use of Asphyxiating, Poisonous or Other Gases, and of Bacteriological Methods of Warfare. Geneva, 17 June 1925, looks a bit ambiguous. "To the end that this prohibition shall be universally accepted as a part of International Law, binding alike the conscience and the practice of nations", but "That the High Contracting Parties ... agree to be bound as between themselves according to the terms of this declaration." with no express statement regarding non-signatories.

But, if a non-contracting party were in, regardless whether all gloves were off for everyone, in the treaties I've seen they were off vis a vis the non-contracting parties.