The T103 observation rounds were designed to give a puff of smoke on impact. The construction is quite complicated - Brandt et al. show a sectioned round in their book on 7.62 NATO cartridges. The bullets most likely contain a sealed pellet of white phosphorous to provide the smoke. If you gently shake genuine cartridges, you can hear the striker mechanism rattle.
The T103 used a FAT1 case (49mm in length) and a modified .30 cal T99 bullet. The T103E1 used the full length case.
Left to right
T103 HS F A 48 * Br cs Ni pr
T103E1 HS F A 51 Br cs and pr, Green PA
Fumer HS (+) FA 71 Br cs and pr, Red PA
X-ray of T103
X-ray of T103E1
T103E1 box (unfortunately opened)
The yellow tipped round described as a fumer is clearly a tracer of some type but very different to the T103E1 - see x-ray (may not be clear in the posted images). The bullet has GMCS jacket and clear tracer compartment at the base, followed by an X-ray opaque core and finally a second compartment in the tip which is not x-ray opaque.
Top - Fumer, bottom T103E1
Dave Hughes in his book on the M16 and its cartridges gives some detail on the fumer effect. He states “A fumer is a method of reducing the base drag of a projectile by injecting the proper amount of heat and mass into the projectile bas region during flight. This could be thought of as somewhat analogous to tracer ammunition as the tracer material is burned in the base area. However, the fumer doesn’t have a luminosity requirement.” As to whether the yellow tipped “tracer” cartridge does not trace or leaves a smoke trace as has been suggested, I don’t have an answer at present.