Almost certainly a Milbro G10 Repeater. One of my friends had one in the late 70's. As you say, they had an incredibly short barrel section that pivoted up to load a pellet or fill the 20 bb chamber above the barrel for 'repeater' use. The gun was cocked by pushing what would be the slide release on a real Colt 45, then pulling back the rear part of the slide. You then completed the cocking by pushing the slide back to complete the compression on the spring. This back & forth movement actually moved a small transfer port in & out of the rubber seal on the back of the barrel which is how it cycled the bbs into the barrel without you having to fiddle with the flip front. I remember that we all had lots of fun hunting tins in the local woods with one of these. Then his brother got an Original G5 and that rather eclipsed the dear old G10

Cheers,
Andy.