That a QB77 is perhaps a Mac-1 prepped QB78?
In case the link does not work:
" What are QB-77, 78 and 22 ?
Industry Brand QB-78 is a .177 caliber, CO2 powered air rifle. It is a copy Crosman 167, made and assembled by Shanghai Airgun Factory in China. QB-77 is the same as QB78, except that it is assembled and inspected by the experienced airgunsmiths of MAC-1 in America. QB-22 is a .22 caliber, and a copy of the Crosman 160. "