Winston Science Fiction was a series, thirty-five, of juveniles published between 1952 and 1961. There was one collection and one non-fiction among them. SF writers mostly, but some more known for other genres. As here(this one and another couple were originally published as by Richard Marsden). All were especially written for the series and distinguished by that rocket on the spine. I frequently looked for that rocket when I was very young. Don’t know that I read all of them back then. Likely not.
Owen and Chuck Spencer should have been suspicious of Dirk Masterson from the start. Owen was the guide for the Time Slip back to the Jurassic era and Chuck was being allowed to go along. Masterson was unco-operative from go, not wanting to prove his identity.
Things only got worse when they went back. Masterson jumps into a jeep and takes off at high speed, crashing into and shoring out the force field that kept the dinosaurs away from them. And the recall wasn’t for a week.
When Masterson and his partner pulled hunting rifles from their truck, The Spencer brothers knew trouble was here. You see, this was a photography only mission. Killing was illegal.
And finally, Chuck discovers mining equipment and dynamite stored in Masterson’s truck.
A nice little juvenile science fiction novel. I did have problems with one plot line, but won’t give that away. Try it and see what you think. They are available as ebooks.