Appellant here seeks reversal of his conviction under an indictment charging him with violation of the Dyer Act.1 He alleges numerous errors in the trial below, but we believe only two merit brief discussion.

Appellant first argues that the trial court erred in refusing to suppress certain testimony concerning evidence allegedly obtained in violation of his Fourth Amendment rights. In early March 1971 a White Freight Lines truck tractor was stolen from a lot in Alabama. On April 10, 1972 witness Moeller, a Los Angeles police officer with over ten years experience investigating heavy equipment thefts, became suspicious of a 'ratty'-appearing truck parked at a truck stop in Lynwood, California notorious as a 'haven . . . for stolen heavy tractor trailor equipment.' Together with an assistant, he investigated the vehicle and obtained its engine, transmission, and license numbers. A later check revealed that the chassis of the truck under investigation came from the vehicle stolen in Alabama and that appellant claimed to be its owner. At his trial appellant moved to suppress all testimony concerning this investigation of the truck's identification numbers, arguing that the procedure used by Moeller constituted an unreasonable search forbidden by the Fourth Amendment. After the required hearing, the trial judge denied the motion.

In a line of cases following United States v. Johnson, 413 F.2d 1396 (1969), aff'd en banc, 431 F.2d 441 (5th Cir. 1970), this Court has upheld the validity of

inspections of motor vehicles performed by police officers, who were entitled to be on the property where the vehicle was located, which in no way damaged the vehicles and were limited to determining the correct identification numbers thereof . . ..

431 F.2d at 441. Appellant attempts to distinguish these authorities by urging that in this case it was necessary for the officer to remove grease or dirt from parts of the truck in order to read the identification numbers. Although there is evidence supporting the government's position that no grease or dirt was in fact removed, the evidentiary conflict is immaterial. In United States v. Polk, 433 F.2d 644 (5th Cir. 1970), we applied the Johnson doctrine even though the inspecting officer had opened the unlocked door of a car to obtain the vehicle inspection number. Assuming arguendo that the officer in this case did remove grease or dirt in order to view the numbers, we can find no basis for distinguishing this action from the actions found permissible in Polk. Hence, we reject appellant's first contention.

Appellant also objects to the trial judge's handling of the recent possession doctrine. Because no objection was raised during the trial, to the extent these complaints relate to the content of the charge we may review only for 'plain error.' Fed.R.Crim.P. 30, 52(b). We find no such error here. Relying on United States v. Hale, 410 F.2d 147 (5th Cir.), cert. denied,396 U.S. 902, 90 S. Ct. 216, 24 L. Ed. 2d 179 (1969), appellant argues that the trial judge erred in failing to find as a matter of law that appellant's possession of the stolen truck was not 'recent.' The Court in Hale explained the applicable law:

What constitutes recent possession is not capable of exact or precise definition; it varies with the circumstances of each case and, for these reasons, it is often said that the question is one of fact for the jury. We agree with these principles. However, it is incumbent upon the trial court to determine whether there exists sufficient evidence to support a finding of recentness by the jury . . .. There is a certain period after which . . . an inference of guilty knowledge cannot be rationally inferred from unexplained possession. Whether this period has expired is a question for the trial court to determine before giving the case to the jury.

410 F.2d at 151. Although the government's proof established appellant's possession of the truck only at a time some thirteen months after the theft, appellant testified that he took possession on a date within six months of the truck's disappearance. This six month period has been found sufficient in similar cases.2 Taking into account all the circumstances of the case, we find ample support for the trial judge's decision allowing the jury to infer guilty knowledge from appellant's unexplained possession of the truck.3

After careful consideration, we find appellant's remaining contentions to be without merit. Accordingly, the judgment is affirmed.