Released sign-to-text checkpoints combine visual and text-side parameters, so intact-checkpoint comparisons cannot identify what drives transfer. We study this confound in low-resource Taiwan Sign Language (TSL) by separating and recombining the visual and mT5 components of Uni-Sign checkpoints trained on Chinese Sign Language (CSL)-to-Chinese and American Sign Language (ASL)-to-English data, then fine-tuning the combinations identically on TSL. The CSL-derived visual initialization improves content-word accuracy by 15.0 points with its released mT5 parameters, but changes accuracy by only -0.5 points with untouched mT5-base, showing that the measured benefit of visual pretraining depends on the text-side initialization. We assess video dependence by replacing target clips with duration-matched held-out clips at evaluation. Further interventions localize much of the accuracy and video-dependence effects to the untied output projection: replacing an ASL-to-English model’s projection with mT5-base’s improves accuracy by 25.0 points and increases sensitivity to clip replacement. Permutation and timing controls show that the effect depends on pretrained row-to-token correspondence and on the projection being present during fine-tuning. Absolute translation quality remains low, so we interpret these experiments diagnostically. The results show that released checkpoints should be analyzed as structured initializations whose components can interact, rather than treated as indivisible evidence of cross-language transfer.