A new 2017 paper on a sex-linked dominant rumpless mutation (reduced tail feathers only, not skeletal, oil gland present):

Wang, Qiong et al. A Novel Sex-Linked Mutant Affecting Tail Formation in Hongshan Chicken. Scientific Reports 7 (2017): 10079. PMC. Web. 19 Sept. 2017.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5577132/

Hongshan chicken photos here:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5577132/figure/Fig1/